Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,wasRe: Bug in slice type
"Ron Adam" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > (I was wondering why list's couldn't have len,min, and max attribute > that are updated when ever the list is modified in place of using > len,min, and max functions? Python's list and, I believe, other builtin roster objects do have a C-level length attribute. For mutables, it is updated. __len__ is just a get of int value the and conversion to int object. Min and max are defined and sensible for only a subset of lists, and when they are, are very seldom need repeated updates. > Would the overhead be that much?) For the fraction of times used, yes. Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Bengt Richter wrote:
> IMO the problem is that the index sign is doing two jobs, which for zero-based
> reverse indexing have to be separate: i.e., to show direction _and_ a _signed_
> offset which needs to be realtive to the direction and base position.
Yes, that's definitely part of it.
> A list-like class, and an option to use a zero-based reverse index will
> illustrate:
>
class Zbrx(object):
>
> ... def __init__(self, value=0):
> ... self.value = value
> ... def __repr__(self): return 'Zbrx(%r)'%self.value
> ... def __sub__(self, other): return Zbrx(self.value - other)
> ... def __add__(self, other): return Zbrx(self.value + other)
> ...
> >>> class Zbrxlist(object):
> ... def normslc(self, slc):
> ... sss = [slc.start, slc.stop, slc.step]
> ... for i,s in enumerate(sss):
> ... if isinstance(s, Zbrx): sss[i] = len(self.value)-1-s.value
> ... return tuple(sss), slice(*sss)
> ... def __init__(self, value):
> ... self.value = value
> ... def __getitem__(self, i):
> ... if isinstance(i, int):
> ... return '[%r]: %r'%(i, self.value[i])
> ... elif isinstance(i, Zbrx):
> ... return '[%r]: %r'%(i, self.value[len(self.value)-1-i.value])
> ... elif isinstance(i, slice):
> ... sss, slc = self.normslc(i)
> ... return '[%r:%r:%r]: %r'%(sss+ (list.__getitem__(self.value,
> slc),))
> ... def __setitem__(self, i, v):
> ... if isinstance(i, int):
> ... list.__setitem__(self, i, v)
> ... elif isinstance(i, slice):
> ... sss, slc = self.normslc(i)
> ... list.__setitem__(self.value, slc, v)
> ... def __repr__(self): return 'Zbrxlist(%r)'%self.value
> ...
> >>> zlast = Zbrx(0)
> >>> zbr10 = Zbrxlist(range(10))
> >>> zbr10[zlast]
> '[Zbrx(0)]: 9'
> >>> zbr10[zlast:]
> '[9:None:None]: [9]'
> >>> zbr10[zlast:zlast] = ['end']
> >>> zbr10
> Zbrxlist([0, 1, 2, 3, 4, 5, 6, 7, 8, 'end', 9])
> >>> ztop = Zbrx(-1)
> >>> zbr10[ztop:ztop] = ['final']
> >>> zbr10
> Zbrxlist([0, 1, 2, 3, 4, 5, 6, 7, 8, 'end', 9, 'final'])
> >>> zbr10[zlast:]
> "[11:None:None]: ['final']"
> >>> zbr10[zlast]
> "[Zbrx(0)]: 'final'"
> >>> zbr10[zlast+1]
> '[Zbrx(1)]: 9'
> >>> zbr10[zlast+2]
> "[Zbrx(2)]: 'end'"
>
> >>> a = Zbrxlist(list('abcde'))
> >>> a
> Zbrxlist(['a', 'b', 'c', 'd', 'e'])
>
> Forgot to provide a __len__ method ;-)
> >>> a[len(a.value):len(a.value)] = ['end']
> >>> a
> Zbrxlist(['a', 'b', 'c', 'd', 'e', 'end'])
>
> lastx refers to the last items by zero-based reverse indexing
> >>> a[lastx]
> "[Zbrx(0)]: 'end'"
> >>> a[lastx:lastx] = ['last']
> >>> a
> Zbrxlist(['a', 'b', 'c', 'd', 'e', 'last', 'end'])
>
> As expected, or do you want to define different semantics?
> You still need to spell len(a) in the slice somehow to indicate
> beond the top. E.g.,
>
> >>> a[lastx-1:lastx-1] = ['final']
> >>> a
> Zbrxlist(['a', 'b', 'c', 'd', 'e', 'last', 'end', 'final'])
>
> Perhaps you can take the above toy and make something that works
> they way you had in mind? Nothing like implementation to give
> your ideas reality ;-)
Thanks, I'll play around with it. ;-)
As you stated before the index is doing two jobs, so limiting it in some
way may be what is needed. Here's a few possible (or impossible) options.
(Some of these aren't pretty.)
* Disallow *all* negative values, use values of start/stop to determine
direction. Indexing from far end needs to be explicit (len(n)-x).
a[len(a):0]reverse order
a[len(a):0:2] reveres order, even items
(I was wondering why list's couldn't have len,min, and max attribute
that are updated when ever the list is modified in place of using
len,min, and max functions? Would the overhead be that much?)
a[len.a:0]
* Disallow negative index's, use negative steps to determine indexing
direction. Order of index's to determine output order.
a[len(a):0:-1] forward order, zero based indexing from end.
a[0:len(a):-1] reverse order, zero based from end.
a[0:1:-1] last item
I works, but single a[-1] is used extremely often. I don't think having
to do a[0:1:-1] would be very popular.
* A reverse index symbol/operator could be used ...
a[~0] -> last item, This works BTW. :-) ~0 == -1
a[~1] -> next to last item
(Could this be related to the original intended use?)
a[~0:~0] slice after end ?. Doesn't work correctly.
What is needed here is to index from the left instead of the right.
a[~0] -> item to left of end gap.
*IF* this could be done; I'm sure there's some reason why this won't
work. ;-), then all indexing operations with '~' could be symmetric with
all positive indexing operations. Then in Python 3k true negative
index's could cause an exception... less bugs I bet. And then negative
steps could reverse lists with a lot less co
Re: Bug in string.find
Terry Reedy wrote: >>b[-1:] = ['Z']# replaces last item >>b[-1:-0] = ['Z'] # this doesn't work >> >>If you are using negative index slices, you need to check for end >>conditions because you can't address the end of the slice in a >>sequential/numerical way. > > OK, now I understand your question, the answer 'get a one's complement > machine', and your point that without a -0 separate from +0, there is a > subtle asymmetry that is easy to overlook. Yes, I don't thinks it falls in the catagory of bugs, probably closer to a minor wart. >>>As I posted before (but perhaps it arrived after you sent this), one >>>number >>>indexing rounds down, introducing a slight asymmetry. >> >>I didn't see that one, > > Perhaps you did not see my other long post, where I drew ascii pictures? Yes, I saw it. I think we are expressing the same things in different ways. > In the US (and UK?), the ground level floor of a multifloor building is the > first floor. In continental Europe (all or just some?), the ground floor > is the ground (effectively zeroth) floor while the first floor up is the > first stage (resting place on the stairway). In the VA Hospital here in Tampa, the ground floor in the front elevator is on the same level as the lobby, while the ground floor in the elevator on the other side of the building is on the floor below. ;-) >>I don't think people would miss negative strides much if they were >>removed. Replacing these case's with reverse() methods shouldn't be that >>difficult. > > Yes, the introduction of reversed partly obsoleted the use of negative > strides, at least outside of its numerical array origin. > > Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Bengt Richter wrote: > On Wed, 31 Aug 2005 14:16:28 GMT, Ron Adam <[EMAIL PROTECTED]> wrote: > [...] > >>The problem with negative index's are that positive index's are zero >>based, but negative index's are 1 based. Which leads to a non >>symmetrical situations. Although it is _way_ too late to try something like this, once upon a time you could have done all of this using the one's complement operator: ~0 does exist and is distinct from 0. So you could talk about a slice: str[4 : ~2] and so on. --Scott David Daniels [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find
"Ron Adam" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > Terry Reedy wrote: >> "Ron Adam" <[EMAIL PROTECTED]> wrote in message >> news:[EMAIL PROTECTED] >>>So how do I express a -0? >> >> >> You just did ;-) but I probably do not know what you mean. > > b[-1:] = ['Z']# replaces last item > b[-1:-0] = ['Z'] # this doesn't work > > If you are using negative index slices, you need to check for end > conditions because you can't address the end of the slice in a > sequential/numerical way. OK, now I understand your question, the answer 'get a one's complement machine', and your point that without a -0 separate from +0, there is a subtle asymmetry that is easy to overlook. >> As I posted before (but perhaps it arrived after you sent this), one >> number >> indexing rounds down, introducing a slight asymmetry. > > I didn't see that one, Perhaps you did not see my other long post, where I drew ascii pictures? In brief: if you draw an axis with ticks, label the tick with ints, and put chars or item references in the space *between* the ticks, then the 'average' coordinate of the stuff between is n.5. The slice interval is n:(n+1), but a single int label has to be either n or (n+1), rounding down or up. And both choices have been made because both have advantages. In the US (and UK?), the ground level floor of a multifloor building is the first floor. In continental Europe (all or just some?), the ground floor is the ground (effectively zeroth) floor while the first floor up is the first stage (resting place on the stairway). > I don't think people would miss negative strides much if they were > removed. Replacing these case's with reverse() methods shouldn't be that > difficult. Yes, the introduction of reversed partly obsoleted the use of negative strides, at least outside of its numerical array origin. Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
On Wed, 31 Aug 2005 14:16:28 GMT, Ron Adam <[EMAIL PROTECTED]> wrote:
[...]
>
>The problem with negative index's are that positive index's are zero
>based, but negative index's are 1 based. Which leads to a non
>symmetrical situations.
>
>Note that you can insert an item before the first item using slices. But
>not after the last item without using len(list) or some value larger
>than len(list).
IMO the problem is that the index sign is doing two jobs, which for zero-based
reverse indexing have to be separate: i.e., to show direction _and_ a _signed_
offset which needs to be realtive to the direction and base position.
A list-like class, and an option to use a zero-based reverse index will
illustrate:
>>> class Zbrx(object):
... def __init__(self, value=0):
... self.value = value
... def __repr__(self): return 'Zbrx(%r)'%self.value
... def __sub__(self, other): return Zbrx(self.value - other)
... def __add__(self, other): return Zbrx(self.value + other)
...
>>> class Zbrxlist(object):
... def normslc(self, slc):
... sss = [slc.start, slc.stop, slc.step]
... for i,s in enumerate(sss):
... if isinstance(s, Zbrx): sss[i] = len(self.value)-1-s.value
... return tuple(sss), slice(*sss)
... def __init__(self, value):
... self.value = value
... def __getitem__(self, i):
... if isinstance(i, int):
... return '[%r]: %r'%(i, self.value[i])
... elif isinstance(i, Zbrx):
... return '[%r]: %r'%(i, self.value[len(self.value)-1-i.value])
... elif isinstance(i, slice):
... sss, slc = self.normslc(i)
... return '[%r:%r:%r]: %r'%(sss+ (list.__getitem__(self.value,
slc),))
... def __setitem__(self, i, v):
... if isinstance(i, int):
... list.__setitem__(self, i, v)
... elif isinstance(i, slice):
... sss, slc = self.normslc(i)
... list.__setitem__(self.value, slc, v)
... def __repr__(self): return 'Zbrxlist(%r)'%self.value
...
>>> zlast = Zbrx(0)
>>> zbr10 = Zbrxlist(range(10))
>>> zbr10[zlast]
'[Zbrx(0)]: 9'
>>> zbr10[zlast:]
'[9:None:None]: [9]'
>>> zbr10[zlast:zlast] = ['end']
>>> zbr10
Zbrxlist([0, 1, 2, 3, 4, 5, 6, 7, 8, 'end', 9])
>>> ztop = Zbrx(-1)
>>> zbr10[ztop:ztop] = ['final']
>>> zbr10
Zbrxlist([0, 1, 2, 3, 4, 5, 6, 7, 8, 'end', 9, 'final'])
>>> zbr10[zlast:]
"[11:None:None]: ['final']"
>>> zbr10[zlast]
"[Zbrx(0)]: 'final'"
>>> zbr10[zlast+1]
'[Zbrx(1)]: 9'
>>> zbr10[zlast+2]
"[Zbrx(2)]: 'end'"
>
> >>> a = list('abcde')
> >>> a[len(a):len(a)] = ['end']
> >>> a
>['a', 'b', 'c', 'd', 'e', 'end']
>
> >>> a[-1:-1] = ['last']
> >>> a
>['a', 'b', 'c', 'd', 'e', 'last', 'end'] # Second to last.
>
> >>> a[100:100] = ['final']
> >>> a
>['a', 'b', 'c', 'd', 'e', 'last', 'end', 'final']
>
>>> a = Zbrxlist(list('abcde'))
>>> a
Zbrxlist(['a', 'b', 'c', 'd', 'e'])
Forgot to provide a __len__ method ;-)
>>> a[len(a.value):len(a.value)] = ['end']
>>> a
Zbrxlist(['a', 'b', 'c', 'd', 'e', 'end'])
lastx refers to the last items by zero-based reverse indexing
>>> a[lastx]
"[Zbrx(0)]: 'end'"
>>> a[lastx:lastx] = ['last']
>>> a
Zbrxlist(['a', 'b', 'c', 'd', 'e', 'last', 'end'])
As expected, or do you want to define different semantics?
You still need to spell len(a) in the slice somehow to indicate
beond the top. E.g.,
>>> a[lastx-1:lastx-1] = ['final']
>>> a
Zbrxlist(['a', 'b', 'c', 'd', 'e', 'last', 'end', 'final'])
Perhaps you can take the above toy and make something that works
they way you had in mind? Nothing like implementation to give
your ideas reality ;-)
Regards,
Bengt Richter
--
http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find
Terry Reedy wrote:
> "Ron Adam" <[EMAIL PROTECTED]> wrote in message
> news:[EMAIL PROTECTED]
>
>>Fredrik Lundh wrote:
>>
>>>Ron Adam wrote:
>>>
The problem with negative index's are that positive index's are zero
based, but negative index's are 1 based. Which leads to a non
symmetrical situations.
>>>
>>>indices point to the "gap" between items, not to the items themselves.
>>
>>So how do I express a -0?
>
>
> You just did ;-) but I probably do not know what you mean.
b[-1:] = ['Z']# replaces last item
b[-1:-0] = ['Z'] # this doesn't work
If you are using negative index slices, you need to check for end
conditions because you can't address the end of the slice in a
sequential/numerical way.
b = list('abcdefg')
for x in range(-len(b),-1):
print b[x:x+2]
['a', 'b']
['b', 'c']
['c', 'd']
['d', 'e']
['e', 'f']
[]
b = list('abcdefg')
for x in range(-len(b),-1):
if x<-2:
print b[x:x+2]
else:
print b[x:]
['a', 'b']
['b', 'c']
['c', 'd']
['d', 'e']
['e', 'f']
['f', 'g']
>> Which should point to the gap after the last item.
>
> The slice index of the gap after the last item is len(seq).
>
>
>>>straight indexing returns the item just to the right of the given gap
>>>(this is
>>>what gives you the perceived assymmetry), slices return all items
>>>between
>>>the given gaps.
>>
>>If this were symmetrical, then positive index's would return the value
>>to the right and negative index's would return the value to the left.
>
> As I posted before (but perhaps it arrived after you sent this), one number
> indexing rounds down, introducing a slight asymmetry.
I didn't see that one, but I agree. Single index's are asymmetric,
positive slices with two index's are again symetric, negative slices
with negative strides or steps are again asymmetric.
>>Have you looked at negative steps? They also are not symmetrical.
>
> ???
print a[4:1:-1]# 6|g| 5|f| 4|e| 3|d| 2|c| 1|b| 0|a| ?
-> edc
print a[-3:-6:-1] # -1|g|-2|f|-3|e|-4|d|-5|c|-6|b|-7|a|-8|
-> edc
# special case '::'
print a[6::-1] # 6|g| 5|f| 4|e| 3|d| 2|c| 1|b| 0|a| ?
-> gfedcba
print a[-1:-8:-1] # -1|g|-2|f|-3|e|-4|d|-5|c|-6|b|-7|a|-8
-> gfedcba
>>All of the following get the center 'd' from the string.
>>
>>a = 'abcdefg'
>>print a[3] # d 4 gaps from beginning
>>print a[-4]# d 5 gaps from end
>
> It is 3 and 4 gaps *from* the left and right end to the left side of the
> 'd'. You can also see the asymmetry as coming from rounding 3.5 and -3.5
> down to 3 and down to -4.
Since single indexing only refers to existing items and aren't used to
insert between items, this still works even with the slight asymmetry.
>>print a[3:4] # d
>>print a[-4:-3] # d
>
> These are is symmetric, as we claimed.
Yes, no problem here except for addressing the -0th (end gap) position
without special casing to either a positive index or a[-n:].
>>print a[3:2:-1]# d These are symetric?!
>>print a[-4:-5:-1] # d
>>print a[3:-5:-1] # d
>>print a[-4:2:-1] # d
>
> The pattern seems to be: left-gap-index : farther-to-left-index : -1 is
> somehow equivalent to left:right, but I never paid much attention to
> strides and don't know the full rule.
a[start:stop:-1]
a[stop:start]# exchange index's
a.reverse() # reverse string
a[4:1:-1]# 6 |g| 5 |f| 4 |e| 3 |d| 2 |c| 1 |b| 0 |a| ?
a[1:4] # ? |a| 0 |b| 1 |c| 2 |d| 3 |e| 4 |f| 5 |g| 6
a.reverse() # -> edc
Notice the index's are 1 less than with positive strides.
> Stride slices are really a different subject from two-gap slicing. They
> were introduced in the early years of Python specificly and only for
> Numerical Python. The rules were those needed specificly for Numerical
> Python arrays. They was made valid for general sequence use only a few
> years ago. I would say that they are only for careful mid-level to expert
> use by those who actually need them for their code.
I'd like to see those use case's. I have a feeling there are probably
better ways to do it now.
Doing a quick search in python24/lib, there are only two places that use
a negative step or stride value to reverse a sequence.
-- PICKLE.PY
return binary[::-1]
ashex = _binascii.hexlify(data[::-1])
I don't think people would miss negative strides much if they were
removed. Replacing these case's with reverse() methods shouldn't be that
difficult.
Cheers,
Ron
> Terry J. Reedy
--
http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find
Dennis Lee Bieber wrote: > On Fri, 02 Sep 2005 00:23:14 GMT, Ron Adam <[EMAIL PROTECTED]> declaimed > the following in comp.lang.python: > > >>So how do I express a -0? Which should point to the gap after the last >>item. >> > > Step one: Obtain a processor that uses ones-complement arithmetic. > Ah, the good old Univac 418 ... [drools into beard and mumbles] > Step two: Port Python to said processor... > Unfortunately the 418 would probably come in at about a micro-pystone, so perhaps we should emulate it on something more modern? regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find
Ron Adam wrote: >> indices point to the "gap" between items, not to the items themselves. > > So how do I express a -0? Which should point to the gap after the last > item. that item doesn't exist when you're doing plain indexing, so being able to express -0 would be pointless. when you're doing slicing, you express it by leaving the value out, or by using len(seq) or (in recent versions) None. >> straight indexing returns the item just to the right of the given gap (this >> is >> what gives you the perceived assymmetry), slices return all items between >> the given gaps. > > If this were symmetrical, then positive index's would return the value > to the right and negative index's would return the value to the left. the gap addressing is symmetrical, but indexing always picks the item to the right. > Have you looked at negative steps? They also are not symmetrical. > print a[3:2:-1]# d These are symetric?! the gap addressing works as before, but to understand exactly what characters you'll get, you have to realize that the slice is really a gap index generator. when you use step=1, you can view slice as a "cut here and cut there, and return what's in between". for other step sizes, you have to think in gap indexes (for which the plain indexing rules apply). and if you know range(), you already know how the indexes are generated for various step sizes. from the range documentation: ... returns a list of plain integers [start, start + step, start + 2 * step, ...]. If step is positive, the last element is the largest start + i * step less than stop; if step is negative, the last element is the largest start + i * step greater than stop. or, in sequence terms (see http://docs.python.org/lib/typesseq.html ) (3) If i or j is negative, the index is relative to the end of the string: len(s) + i or len(s) + j is substituted. ... (5) The slice of s from i to j with step k is defined as the sequence of items with index x = i + n*k for n in the range(0,(j-i)/k). In other words, the indices are i, i+k, i+2*k, i+3*k and so on, stopping when j is reached (but never including j). so in this case, you get >>> 3 + 0*-1 3 >>> 3 + 1*-1 2 # which is your stop condition so a[3:2:-1] is the same as a[3]. > print a[-4:-5:-1] # d same as a[-4] > print a[3:-5:-1] # d now you're mixing addressing modes, which is a great way to confuse yourself. if you normalize the gap indexes (rule 3 above), you'll get a[3:2:-1] which is the same as your earlier example. you can use the "indices" method to let Python do this for you: >>> slice(3,-5,-1).indices(len(a)) (3, 2, -1) >>> range(*slice(3,-5,-1).indices(len(a))) [3] > print a[-4:2:-1] # d same problem here; indices will tell you what that really means: >>> slice(-4,2,-1).indices(len(a)) (3, 2, -1) >>> range(*slice(-4,2,-1).indices(len(a))) [3] same example again, in other words. and same result. > This is why it confuses so many people. It's a shame too, because slice > objects could be so much more useful for indirectly accessing list > ranges. But I think this needs to be fixed first. as everything else in Python, if you use the wrong mental model, things may look "assymmetrical" or "confusing" or "inconsistent". if you look at how things really work, it's usually extremely simple and more often than not internally consistent (since the designers have the "big picture", and knows what they're tried to be consistent with; when slice steps were added, existing slicing rules and range() were the obvious references). it's of course pretty common that people who didn't read the documentation very carefully and therefore adopted the wrong model will insist that Python uses a buggy implementation of their model, rather than a perfectly consistent implementation of the actual model. slices with non-standard step sizes are obviously one such thing, immutable/mutable objects and the exact behaviour of for-else, while-else, and try-else are others. as usual, being able to reset your brain is the only thing that helps. -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find
Ron Adam <[EMAIL PROTECTED]> writes: > All of the following get the center 'd' from the string. > > a = 'abcdefg' > print a[3] # d 4 gaps from beginning > print a[-4]# d 5 gaps from end > print a[3:4] # d > print a[-4:-3] # d > print a[-4:4] # d > print a[3:-3] # d > print a[3:2:-1]# d These are symetric?! > print a[-4:-5:-1] # d > print a[3:-5:-1] # d > print a[-4:2:-1] # d +1 QOTW -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find
"Ron Adam" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > Fredrik Lundh wrote: >> Ron Adam wrote: >>>The problem with negative index's are that positive index's are zero >>>based, but negative index's are 1 based. Which leads to a non >>>symmetrical situations. >> >> indices point to the "gap" between items, not to the items themselves. > > So how do I express a -0? You just did ;-) but I probably do not know what you mean. > Which should point to the gap after the last item. The slice index of the gap after the last item is len(seq). >> straight indexing returns the item just to the right of the given gap >> (this is >> what gives you the perceived assymmetry), slices return all items >> between >> the given gaps. > > If this were symmetrical, then positive index's would return the value > to the right and negative index's would return the value to the left. As I posted before (but perhaps it arrived after you sent this), one number indexing rounds down, introducing a slight asymmetry. > Have you looked at negative steps? They also are not symmetrical. ??? > All of the following get the center 'd' from the string. > > a = 'abcdefg' > print a[3] # d 4 gaps from beginning > print a[-4]# d 5 gaps from end It is 3 and 4 gaps *from* the left and right end to the left side of the 'd'. You can also see the asymmetry as coming from rounding 3.5 and -3.5 down to 3 and down to -4. > print a[3:4] # d > print a[-4:-3] # d These are is symmetric, as we claimed. > print a[-4:4] # d Here you count down past and up past the d. > print a[3:-3] # d Here you count up to and down to the d. The count is one more when you cross the d than when you do not. You do different actions, you get different counts. I would not recommend mixing up and down counting to a beginner, and not down and up counting to anyone who did not absolutely have to. > print a[3:2:-1]# d These are symetric?! > print a[-4:-5:-1] # d > print a[3:-5:-1] # d > print a[-4:2:-1] # d The pattern seems to be: left-gap-index : farther-to-left-index : -1 is somehow equivalent to left:right, but I never paid much attention to strides and don't know the full rule. Stride slices are really a different subject from two-gap slicing. They were introduced in the early years of Python specificly and only for Numerical Python. The rules were those needed specificly for Numerical Python arrays. They was made valid for general sequence use only a few years ago. I would say that they are only for careful mid-level to expert use by those who actually need them for their code. Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find
Fredrik Lundh wrote: > Ron Adam wrote: > > >>The problem with negative index's are that positive index's are zero >>based, but negative index's are 1 based. Which leads to a non >>symmetrical situations. > > > indices point to the "gap" between items, not to the items themselves. So how do I express a -0? Which should point to the gap after the last item. > straight indexing returns the item just to the right of the given gap (this is > what gives you the perceived assymmetry), slices return all items between > the given gaps. If this were symmetrical, then positive index's would return the value to the right and negative index's would return the value to the left. Have you looked at negative steps? They also are not symmetrical. All of the following get the center 'd' from the string. a = 'abcdefg' print a[3] # d 4 gaps from beginning print a[-4]# d 5 gaps from end print a[3:4] # d print a[-4:-3] # d print a[-4:4] # d print a[3:-3] # d print a[3:2:-1]# d These are symetric?! print a[-4:-5:-1] # d print a[3:-5:-1] # d print a[-4:2:-1] # d This is why it confuses so many people. It's a shame too, because slice objects could be so much more useful for indirectly accessing list ranges. But I think this needs to be fixed first. Cheers, Ron -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,wasRe: Bug in slice type
"Stefan Rank" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > on 31.08.2005 16:16 Ron Adam said the following: >> The problem with negative index's are that positive index's are zero >> based, but negative index's are 1 based. Which leads to a non >> symmetrical situations. > > Hear, hear. > > This is, for me, the root of the problem. The root of the problem is the misunderstanding of slice indexes and the symmetry-breaking desire to denote an interval of length 1 by 1 number instead of 2. Someday, I may look at the tutorial to see if I can suggest improvements. In the meanwhile, see Fredrik's reply and my supplement thereto and the additional explanation below. > But changing the whole of Python to the (more natural and consistent) > one-based indexing style, for indexing from left and right, is... > difficult. Consider a mathematical axis |_|_|_|_|... 0 1 2 3 4 The numbers represent points separating unit intervals and representing the total count of intervals from the left. Count 'up' to the right is standard practice. Intervals of length n are denoted by 2 numbers, a:b, where b-a = n. Now consider the 'tick marks' to be gui cursor positions. Characters go in the spaces *between* the cursor. (Fixed versus variable space representations are irrelevant here.) More generally, one can put 'items' or 'item indicators' in the spaces to form general sequences rather than just char strings. It seems convenient to indicate a single char or item with a single number instead of two. We could use the average coordinate, n.5. But that is a nuisance, and the one number representation is about convenience, so we round down or up, depending on the language. Each choice has pluses and minuses; Python rounds down. The axis above and Python iterables are potentially unbounded. But actual strings and sequences are finite and have a right end also. Python then gives the option of counting 'down' from that right end and makes the count negative, as is standard. (But it does not make the string/sequence circular). One can devise slight different sequence models, but the above is the one used by Python. It is consistent and not buggy once understood. I hope this clears some of the confusion seen in this thread. Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find
"Fredrik Lundh" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > [slice] indices point to the "gap" between items, not to the items > themselves. > > positive indices start from the left end, negative indices from the > righept end. > > straight indexing returns the item just to the right of the given gap > (this is > what gives you the perceived assymmetry), slices return all items between > the given gaps. Well said. In some languages, straight indexing returns the item to the left instead. The different between items and gaps in seen in old terminals and older screens versus modern gui screens. Then, a cursur sat on top of or under a character space. Now, a cursur sits between chars. Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find
Ron Adam wrote: > The problem with negative index's are that positive index's are zero > based, but negative index's are 1 based. Which leads to a non > symmetrical situations. indices point to the "gap" between items, not to the items themselves. positive indices start from the left end, negative indices from the righept end. straight indexing returns the item just to the right of the given gap (this is what gives you the perceived assymmetry), slices return all items between the given gaps. -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
> [snipped alot from others about indexing, slicing problems, > and the inadequacy of -1 as Not Found indicator] on 31.08.2005 16:16 Ron Adam said the following: > The problem with negative index's are that positive index's are zero > based, but negative index's are 1 based. Which leads to a non > symmetrical situations. Hear, hear. This is, for me, the root of the problem. But changing the whole of Python to the (more natural and consistent) one-based indexing style, for indexing from left and right, is... difficult. -- http://mail.python.org/mailman/listinfo/python-list
Re: Infinity syntax. Re: Bug in string.find; was...
Bengt Richter wrote: > range(9)[4:-!0:-1] == range(5) > >True > Interesting, but wouldn't that last line be > >>> range(9)[4:-!0:-1] == range(5)[::-1] Ups. Yes of course. > >Life can be simpler with unbound limits. > Hm, is "!0" a di-graph symbol for infinity? > What if we get full unicode on our screens? Should > it be rendered with unichr(0x221e) ? And how should > symbols be keyed in? Is there a standard mnemonic > way of using an ascii keyboard, something like typing > Japanese hiragana in some word processing programs? You can ask questions ;-) > I'm not sure about '!' since it already has some semantic > ties to negation and factorial and execution (not to mention > exclamation ;-) If !0 means infinity, what does !2 mean? > > Just rambling ... ;-) I'm not shure too. Probably Inf as a keyword is a much better choice. The only std-library module I found that used Inf was Decimal where Inf has the same meaning. Inf is quick to write ( just one more character than !0 ) and easy to parse for human readers. Rewriting the above statements/expressions leads to: >>> Inf Inf >>> Inf+1 Inf >>> Inf>n# if n is int True >>> Inf/Inf Traceback (...) ... UndefinedValue >>> Inf - Inf Traceback (...) ... UndefinedValue >>> -Inf -Inf >>> range(9)[4:Inf] == range(9)[4:] True >>> range(9)[4:-Inf:-1] == range(5)[::-1] True IMO it's still consice. Kay -- http://mail.python.org/mailman/listinfo/python-list
Re: Infinity syntax. Re: Bug in string.find; was...
On 31 Aug 2005 07:13:26 -0700, "Kay Schluehr" <[EMAIL PROTECTED]> wrote: >Bengt Richter wrote: > >> How about interpreting seq[i] as an abbreviation of seq[i%len(seq)] ? >> That would give a consitent interpretation of seq[-1] and no errors >> for any value ;-) > >Cool, indexing becomes cyclic by default ;) > >But maybe it's better to define it explicitely: > >seq[!i] = seq[i%len(seq)] > >Well, I don't like the latter definition very much because it >introduces special syntax for __getitem__. A better solution may be the >introduction of new syntax and arithmetics for positive and negative >infinite values. Sequencing has to be adapted to handle them. > >The semantics follows that creating of limits of divergent sequences: > >!0 = lim n > n->infinity > >That enables consistent arithmetics: > >!0+k = lim n+k -> !0 >n->infinity > >!0/k = lim n/k -> !0 for k>0, >n->infinity-!0 for k<0 > ZeroDevisionError for k==0 > > >etc. > >In Python notation: > !0 >!0 !0+1 >!0 !0>n# if n is int >True !0/!0 >Traceback (...) >... >UndefinedValue !0 - !0 >Traceback (...) >... >UndefinedValue -!0 >-!0 range(9)[4:!0] == range(9)[4:] >True range(9)[4:-!0:-1] == range(5) >True Interesting, but wouldn't that last line be >>> range(9)[4:-!0:-1] == range(5)[::-1] >Life can be simpler with unbound limits. Hm, is "!0" a di-graph symbol for infinity? What if we get full unicode on our screens? Should it be rendered with unichr(0x221e) ? And how should symbols be keyed in? Is there a standard mnemonic way of using an ascii keyboard, something like typing Japanese hiragana in some word processing programs? I'm not sure about '!' since it already has some semantic ties to negation and factorial and execution (not to mention exclamation ;-) If !0 means infinity, what does !2 mean? Just rambling ... ;-) Regards, Bengt Richter -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Antoon Pardon wrote:
> Op 2005-08-31, Bengt Richter schreef <[EMAIL PROTECTED]>:
>
>>On 31 Aug 2005 07:26:48 GMT, Antoon Pardon <[EMAIL PROTECTED]> wrote:
>>
>>
>>>Op 2005-08-30, Bengt Richter schreef <[EMAIL PROTECTED]>:
>>>
On 30 Aug 2005 10:07:06 GMT, Antoon Pardon <[EMAIL PROTECTED]> wrote:
>Op 2005-08-30, Terry Reedy schreef <[EMAIL PROTECTED]>:
>
>>"Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message
>>news:[EMAIL PROTECTED]
>>
>>
>>>Really it's x[-1]'s behavior that should go, not find/rfind.
>>
>>I complete disagree, x[-1] as an abbreviation of x[len(x)-1] is extremely
>>useful, especially when 'x' is an expression instead of a name.
>
>I don't think the ability to easily index sequences from the right is
>in dispute. Just the fact that negative numbers on their own provide
>this functionality.
>
>Because I sometimes find it usefull to have a sequence start and
>end at arbitrary indexes, I have written a table class. So I
>can have a table that is indexed from e.g. -4 to +6. So how am
>I supposed to easily get at that last value?
Give it a handy property? E.g.,
table.as_python_list[-1]
>>>
>>>Your missing the point, I probably didn't make it clear.
>>>
>>>It is not about the possibilty of doing such a thing. It is
>>>about python providing a frame for such things that work
>>>in general without the need of extra properties in 'special'
>>>cases.
>>>
>>
>>How about interpreting seq[i] as an abbreviation of seq[i%len(seq)] ?
>>That would give a consitent interpretation of seq[-1] and no errors
>>for any value ;-)
>
>
> But the question was not about having a consistent interpretation for
> -1, but about an easy way to get the last value.
>
> But I like your idea. I just think there should be two differnt ways
> to index. maybe use braces in one case.
>
> seq{i} would be pure indexing, that throws exceptions if you
> are out of bound
>
> seq[i] would then be seq{i%len(seq)}
The problem with negative index's are that positive index's are zero
based, but negative index's are 1 based. Which leads to a non
symmetrical situations.
Note that you can insert an item before the first item using slices. But
not after the last item without using len(list) or some value larger
than len(list).
>>> a = list('abcde')
>>> a[len(a):len(a)] = ['end']
>>> a
['a', 'b', 'c', 'd', 'e', 'end']
>>> a[-1:-1] = ['last']
>>> a
['a', 'b', 'c', 'd', 'e', 'last', 'end'] # Second to last.
>>> a[100:100] = ['final']
>>> a
['a', 'b', 'c', 'd', 'e', 'last', 'end', 'final']
Cheers,
Ron
--
http://mail.python.org/mailman/listinfo/python-list
Infinity syntax. Re: Bug in string.find; was...
Bengt Richter wrote: > How about interpreting seq[i] as an abbreviation of seq[i%len(seq)] ? > That would give a consitent interpretation of seq[-1] and no errors > for any value ;-) Cool, indexing becomes cyclic by default ;) But maybe it's better to define it explicitely: seq[!i] = seq[i%len(seq)] Well, I don't like the latter definition very much because it introduces special syntax for __getitem__. A better solution may be the introduction of new syntax and arithmetics for positive and negative infinite values. Sequencing has to be adapted to handle them. The semantics follows that creating of limits of divergent sequences: !0 = lim n n->infinity That enables consistent arithmetics: !0+k = lim n+k -> !0 n->infinity !0/k = lim n/k -> !0 for k>0, n->infinity-!0 for k<0 ZeroDevisionError for k==0 etc. In Python notation: >>> !0 !0 >>> !0+1 !0 >>> !0>n# if n is int True >>> !0/!0 Traceback (...) ... UndefinedValue >>> !0 - !0 Traceback (...) ... UndefinedValue >>> -!0 -!0 >>> range(9)[4:!0] == range(9)[4:] True >>> range(9)[4:-!0:-1] == range(5) True Life can be simpler with unbound limits. Kay -- http://mail.python.org/mailman/listinfo/python-list
Infinity syntax. Re: Bug in string.find; was...
Bengt Richter wrote: > How about interpreting seq[i] as an abbreviation of seq[i%len(seq)] ? > That would give a consitent interpretation of seq[-1] and no errors > for any value ;-) Cool, indexing becomes cyclic by default ;) But maybe it's better to define it explicitely: seq[!i] = seq[i%len(seq)] Well, I don't like the latter definition very much because it introduces special syntax for __getitem__. A better solution may be the introduction of new syntax and arithmetics for positive and negative infinite values. Sequencing has to be adapted to handle them. The semantics follows that creating of limits of divergent sequences: !0 = lim n n->infinity That enables consistent arithmetics: !0+k = lim n+k -> !0 n->infinity !0/k = lim n/k -> !0 for k>0, n->infinity-!0 for k<0 ZeroDevisionError for k==0 etc. In Python notation: >>> !0 !0 >>> !0+1 !0 >>> !0>n# if n is int True >>> !0/!0 Traceback (...) ... UndefinedValue >>> !0 - !0 Traceback (...) ... UndefinedValue >>> -!0 -!0 >>> range(9)[4:!0] == range(9)[4:] True >>> range(9)[4:-!0:-1] == range(5) True Life can be simpler with unbound limits. Kay -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Paul Rubin wrote: > Not every sequence needs __len__; for example, infinite sequences, or > sequences that implement slicing and subscripts by doing lazy > evaluation of iterators: > > digits_of_pi = memoize(generate_pi_digits()) # 3,1,4,1,5,9,2,... > print digits_of_pi[5] # computes 6 digits and prints '9' > print digits_of_pi($-5) # raises exception Good point. I like the memoize thing, so here is one: class memoize (object): """ Build a sequence from an iterable, evaluating as needed. """ def __init__(self, iterable): self.it = iterable self.known = [] def extend_(self, stop): while len(self.known) < stop: self.known.append(self.it.next()) def __getitem__(self, key): if isinstance(key, (int, long)): self.extend_(key + 1) return self.known[key] elif isinstance(key, slice): start, stop, step = key.start, key.stop, key.step stop = start + 1 + (stop - start - 1) // step * step self.extend_(stop) return self.known[start : stop : step] else: raise TypeError(_type_err_note), "Bad subscript type" -- --Bryan -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Op 2005-08-31, Bengt Richter schreef <[EMAIL PROTECTED]>:
> On 31 Aug 2005 07:26:48 GMT, Antoon Pardon <[EMAIL PROTECTED]> wrote:
>
>>Op 2005-08-30, Bengt Richter schreef <[EMAIL PROTECTED]>:
>>> On 30 Aug 2005 10:07:06 GMT, Antoon Pardon <[EMAIL PROTECTED]> wrote:
>>>
Op 2005-08-30, Terry Reedy schreef <[EMAIL PROTECTED]>:
>
> "Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message
> news:[EMAIL PROTECTED]
>
>> Really it's x[-1]'s behavior that should go, not find/rfind.
>
> I complete disagree, x[-1] as an abbreviation of x[len(x)-1] is extremely
> useful, especially when 'x' is an expression instead of a name.
I don't think the ability to easily index sequences from the right is
in dispute. Just the fact that negative numbers on their own provide
this functionality.
Because I sometimes find it usefull to have a sequence start and
end at arbitrary indexes, I have written a table class. So I
can have a table that is indexed from e.g. -4 to +6. So how am
I supposed to easily get at that last value?
>>> Give it a handy property? E.g.,
>>>
>>> table.as_python_list[-1]
>>
>>Your missing the point, I probably didn't make it clear.
>>
>>It is not about the possibilty of doing such a thing. It is
>>about python providing a frame for such things that work
>>in general without the need of extra properties in 'special'
>>cases.
>>
> How about interpreting seq[i] as an abbreviation of seq[i%len(seq)] ?
> That would give a consitent interpretation of seq[-1] and no errors
> for any value ;-)
But the question was not about having a consistent interpretation for
-1, but about an easy way to get the last value.
But I like your idea. I just think there should be two differnt ways
to index. maybe use braces in one case.
seq{i} would be pure indexing, that throws exceptions if you
are out of bound
seq[i] would then be seq{i%len(seq)}
--
Antoon Pardon
--
http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
On 31 Aug 2005 07:26:48 GMT, Antoon Pardon <[EMAIL PROTECTED]> wrote: >Op 2005-08-30, Bengt Richter schreef <[EMAIL PROTECTED]>: >> On 30 Aug 2005 10:07:06 GMT, Antoon Pardon <[EMAIL PROTECTED]> wrote: >> >>>Op 2005-08-30, Terry Reedy schreef <[EMAIL PROTECTED]>: "Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message news:[EMAIL PROTECTED] > Really it's x[-1]'s behavior that should go, not find/rfind. I complete disagree, x[-1] as an abbreviation of x[len(x)-1] is extremely useful, especially when 'x' is an expression instead of a name. >>> >>>I don't think the ability to easily index sequences from the right is >>>in dispute. Just the fact that negative numbers on their own provide >>>this functionality. >>> >>>Because I sometimes find it usefull to have a sequence start and >>>end at arbitrary indexes, I have written a table class. So I >>>can have a table that is indexed from e.g. -4 to +6. So how am >>>I supposed to easily get at that last value? >> Give it a handy property? E.g., >> >> table.as_python_list[-1] > >Your missing the point, I probably didn't make it clear. > >It is not about the possibilty of doing such a thing. It is >about python providing a frame for such things that work >in general without the need of extra properties in 'special' >cases. > How about interpreting seq[i] as an abbreviation of seq[i%len(seq)] ? That would give a consitent interpretation of seq[-1] and no errors for any value ;-) Regards, Bengt Richter -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Op 2005-08-30, Bengt Richter schreef <[EMAIL PROTECTED]>: > On 30 Aug 2005 10:07:06 GMT, Antoon Pardon <[EMAIL PROTECTED]> wrote: > >>Op 2005-08-30, Terry Reedy schreef <[EMAIL PROTECTED]>: >>> >>> "Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message >>> news:[EMAIL PROTECTED] >>> Really it's x[-1]'s behavior that should go, not find/rfind. >>> >>> I complete disagree, x[-1] as an abbreviation of x[len(x)-1] is extremely >>> useful, especially when 'x' is an expression instead of a name. >> >>I don't think the ability to easily index sequences from the right is >>in dispute. Just the fact that negative numbers on their own provide >>this functionality. >> >>Because I sometimes find it usefull to have a sequence start and >>end at arbitrary indexes, I have written a table class. So I >>can have a table that is indexed from e.g. -4 to +6. So how am >>I supposed to easily get at that last value? > Give it a handy property? E.g., > > table.as_python_list[-1] Your missing the point, I probably didn't make it clear. It is not about the possibilty of doing such a thing. It is about python providing a frame for such things that work in general without the need of extra properties in 'special' cases. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Op 2005-08-30, Steve Holden schreef <[EMAIL PROTECTED]>:
> Antoon Pardon wrote:
>> Op 2005-08-29, Steve Holden schreef <[EMAIL PROTECTED]>:
>>
>>>Antoon Pardon wrote:
>>>
Op 2005-08-27, Steve Holden schreef <[EMAIL PROTECTED]>:
>If you want an exception from your code when 'w' isn't in the string you
>should consider using index() rather than find.
Sometimes it is convenient to have the exception thrown at a later
time.
>Otherwise, whatever find() returns you will have to have an "if" in
>there to handle the not-found case.
And maybe the more convenient place for this "if" is in a whole different
part of your program, a part where using -1 as an invalid index isn't
at all obvious.
>This just sounds like whining to me. If you want to catch errors, use a
>function that will raise an exception rather than relying on the
>invalidity of the result.
You always seem to look at such things in a very narrow scope. You never
seem to consider that various parts of a program have to work together.
>>>
>>>Or perhaps it's just that I try not to mix parts inappropriately.
>>
>>
>> I didn't know it was inappropriately to mix certain parts. Can you
>> give a list of modules in the standard list I shouldn't mix.
>>
>>
So what happens if you have a module that is collecting string-index
pair, colleted from various other parts. In one part you
want to select the last letter, so you pythonically choose -1 as
index. In an other part you get a result of find and are happy
with -1 as an indictation for an invalid index. Then these
data meet.
>>>
>>>That's when debugging has to start. Mixing data of such types is
>>>somewhat inadvisable, don't you agree?
>>
>>
>> The type of both data is the same, it is a string-index pair in
>> both cases. The problem is that a module from the standard lib
>> uses a certain value to indicate an illegal index, that has
>> a very legal value in python in general.
>>
> Since you are clearly feeling pedantic enough to beat this one to death
> with a 2 x 4 please let me substitute "usages" for "types".
But it's not my usage but python's usage.
> In the case of a find() result -1 *isn't* a string index, it's a failure
> flag. Which is precisely why it should be filtered out of any set of
> indexes. once it's been inserted it can no longer be distinguished as a
> failure indication.
Which is precisely why it was such a bad choice in the first place.
If I need to write code like this:
var = str.find('.')
if var == -1:
var = None
each time I want to store an index for later use, then surely '-1'
shouldn't have been used here.
>>>I suppose I can't deny that people do things like that, myself included,
>>
>>
>> It is not about what people do. If this was about someone implementing
>> find himself and using -1 as an illegal index, I would certainly agree
>> that it was inadvisable to do so. Yet when this is what python with
>> its libary offers the programmer, you seem reluctant find fault with
>> it.
> I've already admitted that the choice of -1 as a return value wasn't
> smart. However you appear to be saying that it's sensible to mix return
> values from find() with general-case index values.
I'm saying it should be possible without a problem. It is poor design
to return a legal value as an indication for an error flag.
> I'm saying that you
> should do so only with caution. The fact that the naiive user will often
> not have the wisdom to apply such caution is what makes a change desirable.
I don't think it is naive, if you expect that no legal value will be
returned as an error flag.
>>>but mixing data sets where -1 is variously an error flag and a valid
>>>index is only going to lead to trouble when the combined data is used.
>>
>>
>> Yet this is what python does. Using -1 variously as an error flag and
>> a valid index and when people complain about that, you say it sounds like
>> whining.
>>
> What I am trying to say is that this doesn't make sense: if you want to
> combine find() results with general-case indexes (i.e. both positive and
> negative index values) it behooves you to strip out the -1's before you
> do so. Any other behaviour is asking for trouble.
I would say that choosing this particular return value as an error flag
was asking for trouble. My impression is that you are putting more
blame on the programmer which fails to take corrective action, instead
of on the design of find, which makes that corrective action needed
in the first place.
--
Antoon Pardon
--
http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
On 30 Aug 2005 10:07:06 GMT, Antoon Pardon <[EMAIL PROTECTED]> wrote: >Op 2005-08-30, Terry Reedy schreef <[EMAIL PROTECTED]>: >> >> "Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message >> news:[EMAIL PROTECTED] >> >>> Really it's x[-1]'s behavior that should go, not find/rfind. >> >> I complete disagree, x[-1] as an abbreviation of x[len(x)-1] is extremely >> useful, especially when 'x' is an expression instead of a name. > >I don't think the ability to easily index sequences from the right is >in dispute. Just the fact that negative numbers on their own provide >this functionality. > >Because I sometimes find it usefull to have a sequence start and >end at arbitrary indexes, I have written a table class. So I >can have a table that is indexed from e.g. -4 to +6. So how am >I supposed to easily get at that last value? Give it a handy property? E.g., table.as_python_list[-1] Regards, Bengt Richter -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
On Tue, 30 Aug 2005 11:56:24 GMT, Bryan Olson <[EMAIL PROTECTED]> wrote: >Robert Kern wrote: > > Bryan Olson wrote: > > > > > >> Currently, user-defined classes can implement Python > >> subscripting and slicing without implementing Python's len() > >> function. In our proposal, the '$' symbol stands for the > >> sequence's length, so classes must be able to report their > >> length in order for $ to work within their slices and > >> indexes. > >> > >> Specifically, to support new-style slicing, a class that > >> accepts index or slice arguments to any of: > >> > >> __getitem__ > >> __setitem__ > >> __delitem__ > >> __getslice__ > >> __setslice__ > >> __delslice__ > >> > >> must also consistently implement: > >> > >> __len__ > >> > >> Sane programmers already follow this rule. > > > > > > Incorrect. Some sane programmers have multiple dimensions they need to > > index. > > > > from Numeric import * > > A = array([[0, 1], [2, 3], [4, 5]]) > > A[$-1, $-1] > > > > The result of len(A) has nothing to do with the second $. > >I think you have a good observation there, but I'll stand by my >correctness. > >My initial post considered re-interpreting tuple arguments, but >I abandoned that alternative after Steven Bethard pointed out >how much code it would break. Modules/classes would remain free >to interpret tuple arguments in any way they wish. I don't think >my proposal breaks any sane existing code. > >Going forward, I would advocate that user classes which >implement their own kind of subscripting adopt the '$' syntax, >and interpret it as consistently as possible. For example, they >could respond to __len__() by returning a type that supports the >"Emulating numeric types" methods from the Python Language >Reference 3.3.7, and also allows the class's methods to tell >that it stands for the length of the dimension in question. > > (OTTOMH ;-) Perhaps the slice triple could be extended with a flag indicating which of the other elements should have $ added to it, and $ would take meaning from the subarray being indexed, not the whole. E.g., arr.[1:$-1, $-5:$-2] would call arr.__getitem__((slice(1,-1,None,STOP), slice(-5,-2,None,START|STOP)) (Hypothesizing bitmask constants START and STOP) Regards, Bengt Richter -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
[EMAIL PROTECTED] (Bengt Richter) writes: > What about if when brackets trail as if attributes, it means > your-style slicing written with colons instead of semicolons? > > sequence.[start : stop : step] This is nice. It gets rid of the whole $1,$2,etc syntax as well. -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
On Tue, 30 Aug 2005 08:53:27 GMT, Bryan Olson <[EMAIL PROTECTED]> wrote: [...] >Specification > > We propose a new style of slicing and indexing for Python > sequences. Instead of: > > sequence[start : stop : step] > > new-style slicing uses the syntax: > > sequence[start ; stop ; step] > I don't mind the semantics, but I don't like the semicolons ;-) What about if when brackets trail as if attributes, it means your-style slicing written with colons instead of semicolons? sequence.[start : stop : step] I think that would just be a tweak on the trailer syntax. I just really dislike the semicolons ;-) Regards, Bengt Richter -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Antoon Pardon wrote: > Op 2005-08-29, Steve Holden schreef <[EMAIL PROTECTED]>: > >>Antoon Pardon wrote: >> >>>Op 2005-08-27, Steve Holden schreef <[EMAIL PROTECTED]>: >>> >>> If you want an exception from your code when 'w' isn't in the string you should consider using index() rather than find. >>> >>> >>>Sometimes it is convenient to have the exception thrown at a later >>>time. >>> >>> >>> Otherwise, whatever find() returns you will have to have an "if" in there to handle the not-found case. >>> >>> >>>And maybe the more convenient place for this "if" is in a whole different >>>part of your program, a part where using -1 as an invalid index isn't >>>at all obvious. >>> >>> >>> This just sounds like whining to me. If you want to catch errors, use a function that will raise an exception rather than relying on the invalidity of the result. >>> >>> >>>You always seem to look at such things in a very narrow scope. You never >>>seem to consider that various parts of a program have to work together. >>> >> >>Or perhaps it's just that I try not to mix parts inappropriately. > > > I didn't know it was inappropriately to mix certain parts. Can you > give a list of modules in the standard list I shouldn't mix. > > >>>So what happens if you have a module that is collecting string-index >>>pair, colleted from various other parts. In one part you >>>want to select the last letter, so you pythonically choose -1 as >>>index. In an other part you get a result of find and are happy >>>with -1 as an indictation for an invalid index. Then these >>>data meet. >>> >> >>That's when debugging has to start. Mixing data of such types is >>somewhat inadvisable, don't you agree? > > > The type of both data is the same, it is a string-index pair in > both cases. The problem is that a module from the standard lib > uses a certain value to indicate an illegal index, that has > a very legal value in python in general. > Since you are clearly feeling pedantic enough to beat this one to death with a 2 x 4 please let me substitute "usages" for "types". In the case of a find() result -1 *isn't* a string index, it's a failure flag. Which is precisely why it should be filtered out of any set of indexes. once it's been inserted it can no longer be distinguished as a failure indication. > >>I suppose I can't deny that people do things like that, myself included, > > > It is not about what people do. If this was about someone implementing > find himself and using -1 as an illegal index, I would certainly agree > that it was inadvisable to do so. Yet when this is what python with > its libary offers the programmer, you seem reluctant find fault with > it. > I've already admitted that the choice of -1 as a return value wasn't smart. However you appear to be saying that it's sensible to mix return values from find() with general-case index values. I'm saying that you should do so only with caution. The fact that the naiive user will often not have the wisdom to apply such caution is what makes a change desirable. > >>but mixing data sets where -1 is variously an error flag and a valid >>index is only going to lead to trouble when the combined data is used. > > > Yet this is what python does. Using -1 variously as an error flag and > a valid index and when people complain about that, you say it sounds like > whining. > What I am trying to say is that this doesn't make sense: if you want to combine find() results with general-case indexes (i.e. both positive and negative index values) it behooves you to strip out the -1's before you do so. Any other behaviour is asking for trouble. regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
On Tue, 30 Aug 2005 08:53:27 GMT, Bryan Olson <[EMAIL PROTECTED]> wrote: > Specifically, to support new-style slicing, a class that > accepts index or slice arguments to any of: > > __getitem__ > __setitem__ > __delitem__ > __getslice__ > __setslice__ > __delslice__ > > must also consistently implement: > > __len__ > > Sane programmers already follow this rule. Wouldn't it be more sensible to have an abstract IndexedCollection superclass, which imlements all the slicing stuff, then when someone writes their own collection class they just have to implement __len__ and __getitem__ and slicing works automatically? -- Email: zen19725 at zen dot co dot uk -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Bryan Olson wrote: > Robert Kern wrote: > > from Numeric import * > > A = array([[0, 1], [2, 3], [4, 5]]) > > A[$-1, $-1] > > > > The result of len(A) has nothing to do with the second $. > > I think you have a good observation there, but I'll stand by my > correctness. len() cannot be used to determine the value of $ in the context of multiple dimensions. > My initial post considered re-interpreting tuple arguments, but > I abandoned that alternative after Steven Bethard pointed out > how much code it would break. Modules/classes would remain free > to interpret tuple arguments in any way they wish. I don't think > my proposal breaks any sane existing code. What it does do is provide a second way to do indexing from the end that can't be extended to multiple dimensions. > Going forward, I would advocate that user classes which > implement their own kind of subscripting adopt the '$' syntax, > and interpret it as consistently as possible. How? You haven't proposed how an object gets the information that $-syntax is being used. You've proposed a syntax and some semantics; you also need to flesh out the pragmatics. > For example, they > could respond to __len__() by returning a type that supports the > "Emulating numeric types" methods from the Python Language > Reference 3.3.7, and also allows the class's methods to tell > that it stands for the length of the dimension in question. I have serious doubts about __len__() returning anything but a bona-fide integer. We shouldn't need to use incredible hacks like that to support a core language feature. -- Robert Kern [EMAIL PROTECTED] "In the fields of hell where the grass grows high Are the graves of dreams allowed to die." -- Richard Harter -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Robert Kern wrote: > Bryan Olson wrote: > > >> Currently, user-defined classes can implement Python >> subscripting and slicing without implementing Python's len() >> function. In our proposal, the '$' symbol stands for the >> sequence's length, so classes must be able to report their >> length in order for $ to work within their slices and >> indexes. >> >> Specifically, to support new-style slicing, a class that >> accepts index or slice arguments to any of: >> >> __getitem__ >> __setitem__ >> __delitem__ >> __getslice__ >> __setslice__ >> __delslice__ >> >> must also consistently implement: >> >> __len__ >> >> Sane programmers already follow this rule. > > > Incorrect. Some sane programmers have multiple dimensions they need to > index. > > from Numeric import * > A = array([[0, 1], [2, 3], [4, 5]]) > A[$-1, $-1] > > The result of len(A) has nothing to do with the second $. I think you have a good observation there, but I'll stand by my correctness. My initial post considered re-interpreting tuple arguments, but I abandoned that alternative after Steven Bethard pointed out how much code it would break. Modules/classes would remain free to interpret tuple arguments in any way they wish. I don't think my proposal breaks any sane existing code. Going forward, I would advocate that user classes which implement their own kind of subscripting adopt the '$' syntax, and interpret it as consistently as possible. For example, they could respond to __len__() by returning a type that supports the "Emulating numeric types" methods from the Python Language Reference 3.3.7, and also allows the class's methods to tell that it stands for the length of the dimension in question. -- --Bryan -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Antoon Pardon wrote: > Op 2005-08-30, Robert Kern schreef <[EMAIL PROTECTED]>: > >>Bryan Olson wrote: >> >>> Currently, user-defined classes can implement Python >>> subscripting and slicing without implementing Python's len() >>> function. In our proposal, the '$' symbol stands for the >>> sequence's length, so classes must be able to report their >>> length in order for $ to work within their slices and >>> indexes. >>> >>> Specifically, to support new-style slicing, a class that >>> accepts index or slice arguments to any of: >>> >>> __getitem__ >>> __setitem__ >>> __delitem__ >>> __getslice__ >>> __setslice__ >>> __delslice__ >>> >>> must also consistently implement: >>> >>> __len__ >>> >>> Sane programmers already follow this rule. >> >>Incorrect. Some sane programmers have multiple dimensions they need to >>index. > > I don't see how that contradicts Bryan's statement. > >> from Numeric import * >> A = array([[0, 1], [2, 3], [4, 5]]) >> A[$-1, $-1] >> >>The result of len(A) has nothing to do with the second $. > > But that is irrelevant to the fact wether or not sane > programmes follow Bryan's stated rule. That the second > $ has nothing to do with len(A), doesn't contradict > __len__ has to be implemented nor that sane programers > already do. Except that the *consistent* implementation is supposed to support the interpretation of $. It clearly can't for multiple dimensions. -- Robert Kern [EMAIL PROTECTED] "In the fields of hell where the grass grows high Are the graves of dreams allowed to die." -- Richard Harter -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Op 2005-08-30, Robert Kern schreef <[EMAIL PROTECTED]>: > Bryan Olson wrote: > >> Currently, user-defined classes can implement Python >> subscripting and slicing without implementing Python's len() >> function. In our proposal, the '$' symbol stands for the >> sequence's length, so classes must be able to report their >> length in order for $ to work within their slices and >> indexes. >> >> Specifically, to support new-style slicing, a class that >> accepts index or slice arguments to any of: >> >> __getitem__ >> __setitem__ >> __delitem__ >> __getslice__ >> __setslice__ >> __delslice__ >> >> must also consistently implement: >> >> __len__ >> >> Sane programmers already follow this rule. > > Incorrect. Some sane programmers have multiple dimensions they need to > index. I don't see how that contradicts Bryan's statement. > from Numeric import * > A = array([[0, 1], [2, 3], [4, 5]]) > A[$-1, $-1] > > The result of len(A) has nothing to do with the second $. But that is irrelevant to the fact wether or not sane programmes follow Bryan's stated rule. That the second $ has nothing to do with len(A), doesn't contradict __len__ has to be implemented nor that sane programers already do. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Bryan Olson wrote: > Currently, user-defined classes can implement Python > subscripting and slicing without implementing Python's len() > function. In our proposal, the '$' symbol stands for the > sequence's length, so classes must be able to report their > length in order for $ to work within their slices and > indexes. > > Specifically, to support new-style slicing, a class that > accepts index or slice arguments to any of: > > __getitem__ > __setitem__ > __delitem__ > __getslice__ > __setslice__ > __delslice__ > > must also consistently implement: > > __len__ > > Sane programmers already follow this rule. Incorrect. Some sane programmers have multiple dimensions they need to index. from Numeric import * A = array([[0, 1], [2, 3], [4, 5]]) A[$-1, $-1] The result of len(A) has nothing to do with the second $. -- Robert Kern [EMAIL PROTECTED] "In the fields of hell where the grass grows high Are the graves of dreams allowed to die." -- Richard Harter -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Op 2005-08-30, Terry Reedy schreef <[EMAIL PROTECTED]>: > > "Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message > news:[EMAIL PROTECTED] > >> Really it's x[-1]'s behavior that should go, not find/rfind. > > I complete disagree, x[-1] as an abbreviation of x[len(x)-1] is extremely > useful, especially when 'x' is an expression instead of a name. I don't think the ability to easily index sequences from the right is in dispute. Just the fact that negative numbers on their own provide this functionality. Because I sometimes find it usefull to have a sequence start and end at arbitrary indexes, I have written a table class. So I can have a table that is indexed from e.g. -4 to +6. So how am I supposed to easily get at that last value? -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Bryan Olson <[EMAIL PROTECTED]> writes: > Specifically, to support new-style slicing, a class that > accepts index or slice arguments to any of: > > __getitem__ > __setitem__ > __delitem__ > __getslice__ > __setslice__ > __delslice__ > > must also consistently implement: > > __len__ > > Sane programmers already follow this rule. It should be ok to use new-style slicing without implementing __len__ as long as you don't use $ in any slices. Using $ in a slice without __len__ would throw a runtime error. I expect using negative subscripts in old-style slices on objects with no __len__ also throws an error. Not every sequence needs __len__; for example, infinite sequences, or sequences that implement slicing and subscripts by doing lazy evaluation of iterators: digits_of_pi = memoize(generate_pi_digits()) # 3,1,4,1,5,9,2,... print digits_of_pi[5] # computes 6 digits and prints '9' print digits_of_pi($-5) # raises exception -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Terry Reedy wrote:
> "Paul Rubin" wrote:
>
>>Really it's x[-1]'s behavior that should go, not find/rfind.
>
> I complete disagree, x[-1] as an abbreviation of x[len(x)-1] is
extremely
> useful, especially when 'x' is an expression instead of a name.
Hear us out; your disagreement might not be so complete as you
think. From-the-far-end indexing is too useful a feature to
trash. If you look back several posts, you'll see that the
suggestion here is that the index expression should explicitly
call for it, rather than treat negative integers as a special
case.
I wrote up and sent off my proposal, and once the PEP-Editors
respond, I'll be pitching it on the python-dev list. Below is
the version I sent (not yet a listed PEP).
--
--Bryan
PEP: -1
Title: Improved from-the-end indexing and slicing
Version: $Revision: 1.00 $
Last-Modified: $Date: 2005/08/26 00:00:00 $
Author: Bryan G. Olson <[EMAIL PROTECTED]>
Status: Draft
Type: Standards Track
Content-Type: text/plain
Created: 26 Aug 2005
Post-History:
Abstract
To index or slice a sequence from the far end, we propose
using a symbol, '$', to stand for the length, instead of
Python's current special-case interpretation of negative
subscripts. Where Python currently uses:
sequence[-i]
We propose:
sequence[$ - i]
Python's treatment of negative indexes as offsets from the
high end of a sequence causes minor obvious problems and
major subtle ones. This PEP proposes a consistent meaning
for indexes, yet still supports from-the-far-end
indexing. Use of new syntax avoids breaking existing code.
Specification
We propose a new style of slicing and indexing for Python
sequences. Instead of:
sequence[start : stop : step]
new-style slicing uses the syntax:
sequence[start ; stop ; step]
It works like current slicing, except that negative start or
stop values do not trigger from-the-high-end interpretation.
Omissions and 'None' work the same as in old-style slicing.
Within the square-brackets, the '$' symbol stands for the
length of the sequence. One can index from the high end by
subtracting the index from '$'. Instead of:
seq[3 : -4]
we write:
seq[3 ; $ - 4]
When square-brackets appear within other square-brackets,
the inner-most bracket-pair determines which sequence '$'
describes. The length of the next-outer sequence is denoted
by '$1', and the next-out after than by '$2', and so on. The
symbol '$0' behaves identically to '$'. Resolution of $x is
syntactic; a callable object invoked within square brackets
cannot use the symbol to examine the context of the call.
The '$' notation also works in simple (non-slice) indexing.
Instead of:
seq[-2]
we write:
seq[$ - 2]
If we did not care about backward compatibility, new-style
slicing would define seq[-2] to be out-of-bounds. Of course
we do care about backward compatibility, and rejecting
negative indexes would break way too much code. For now,
simple indexing with a negative subscript (and no '$') must
continue to index from the high end, as a deprecated
feature. The presence of '$' always indicates new-style
indexing, so a programmer who needs a negative index to
trigger a range error can write:
seq[($ - $) + index]
Motivation
From-the-far-end indexing is such a useful feature that we
cannot reasonably propose its removal; nevertheless Python's
current method, which is to treat a range of negative
indexes as special cases, is warty. The wart bites novice or
imperfect Pythoners by not raising an exceptions when they
need to know about a bug. For example, the following code
prints 'y' with no sign of error:
s = 'buggy'
print s[s.find('w')]
The wart becomes an even bigger problem with more
sophisticated use of Python sequences. What is the 'stop'
value for a slice when the step is negative and the slice
includes the zero index? An instance of Python's slice type
will report that the stop value is -1, but if we use this
stop value to slice, it gets misinterpreted as the last
index in the sequence. Here's an example:
class BuggerAll:
def __init__(self, somelist):
self.sequence = somelist[:]
def __getitem__(self, key):
if isinstance(key, slice):
start, stop, step = key.indices(len(self.sequence))
# print 'Slice says start, stop, step are:', start,
stop, step
return self.sequence[start : stop : step]
print range(10) [None : None : -2]
print BuggerAll(range(10))[None : None : -2]
The above prints:
[9, 7, 5, 3, 1]
[]
Un-commenting the print state
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
"Terry Reedy" <[EMAIL PROTECTED]> writes: > > Really it's x[-1]'s behavior that should go, not find/rfind. > > I complete disagree, x[-1] as an abbreviation of x[len(x)-1] is extremely > useful, especially when 'x' is an expression instead of a name. There are other abbreviations possible, for example the one in the proposed PEP at the beginning of this thread. > But even > if -1 were not a legal subscript, I would still consider it a design error > for Python to mistype a non-numeric singleton indicator as an int. OK, .find should return None if the string is not found. -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
"Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message news:[EMAIL PROTECTED] > Really it's x[-1]'s behavior that should go, not find/rfind. I complete disagree, x[-1] as an abbreviation of x[len(x)-1] is extremely useful, especially when 'x' is an expression instead of a name. But even if -1 were not a legal subscript, I would still consider it a design error for Python to mistype a non-numeric singleton indicator as an int. Such mistyping is only necessary in a language like C that requires all return values to be of the same type, even when the 'return value' is not really a return value but an error signal. Python does not have that typing restriction and should not act as if it does by copying C. > Will socket.connect_ex also go? Not familiar with it. > How about dict.get? A default value is not necessarily an error indicator. One can regard a dict that is 'getted' as an infinite dict matching all keys with the default except for a finite subset of keys, as recorded in the dict. If the default is to be regarded a 'Nothing to return' indicator, then that indicator *must not* be in the dict. A recommended idiom is to then create a new, custom subset of object which *cannot* be a value in the dict. Return values can they safely be compared with that indicator by using the 'is' operator. In either case, .get is significantly different from .find. Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Op 2005-08-29, Steven Bethard schreef <[EMAIL PROTECTED]>:
> Antoon Pardon wrote:
>> I think a properly implented find is better than an index.
>
> See the current thread in python-dev[1], which proposes a new method,
> str.partition(). I believe that Raymond Hettinger has shown that almost
> all uses of str.find() can be more clearly be represented with his
> proposed function.
Do we really need this? As far as I understand most of this
functionality is already provided by str.split and str.rsplit
I think adding an optional third parameter 'full=False' to these
methods, would be all that is usefull here. If full was set
to True, split and rsplit would enforce that a list with
maxsplit + 1 elements was returned, filling up the list with
None's if necessary.
head, sep, tail = str.partion(sep)
would then almost be equivallent to
head, tail = str.find(sep, 1, True)
Code like the following:
head, found, tail = result.partition(' ')
if not found:
break
result = head + tail
Could be replaced by:
head, tail = result.split(' ', 1, full = True)
if tail is None
break
result = head + tail
I also think that code like this:
while tail:
head, _, tail = tail.partition('.')
mname = "%s.%s" % (m.__name__, head)
m = self.import_it(head, mname, m)
...
Would probably better be written as follows:
for head in tail.split('.'):
mname = "%s.%s" % (m.__name__, head)
m = self.import_it(head, mname, m)
...
Unless I'm missing something.
--
Antoon Pardon
[1]http://mail.python.org/pipermail/python-dev/2005-August/055781.html
--
http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Steve Holden wrote: > I'm all in favor of discussions to make 3.0 a better > language. This one should definitely be two-phase. First, the non-code- breaking change that replaces-and-deprecates the warty handling of negative indexes, and later the removal of the old style. For the former, there's no need to wait for a X.0 release; for the latter, 3.0 may be too early. The draft PEP went to the PEP editors a couple days ago. Haven't heard back yet. -- --Bryan -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Op 2005-08-29, Steve Holden schreef <[EMAIL PROTECTED]>: > Antoon Pardon wrote: >> Op 2005-08-27, Steve Holden schreef <[EMAIL PROTECTED]>: >> >>>If you want an exception from your code when 'w' isn't in the string you >>>should consider using index() rather than find. >> >> >> Sometimes it is convenient to have the exception thrown at a later >> time. >> >> >>>Otherwise, whatever find() returns you will have to have an "if" in >>>there to handle the not-found case. >> >> >> And maybe the more convenient place for this "if" is in a whole different >> part of your program, a part where using -1 as an invalid index isn't >> at all obvious. >> >> >>>This just sounds like whining to me. If you want to catch errors, use a >>>function that will raise an exception rather than relying on the >>>invalidity of the result. >> >> >> You always seem to look at such things in a very narrow scope. You never >> seem to consider that various parts of a program have to work together. >> > Or perhaps it's just that I try not to mix parts inappropriately. I didn't know it was inappropriately to mix certain parts. Can you give a list of modules in the standard list I shouldn't mix. >> So what happens if you have a module that is collecting string-index >> pair, colleted from various other parts. In one part you >> want to select the last letter, so you pythonically choose -1 as >> index. In an other part you get a result of find and are happy >> with -1 as an indictation for an invalid index. Then these >> data meet. >> > That's when debugging has to start. Mixing data of such types is > somewhat inadvisable, don't you agree? The type of both data is the same, it is a string-index pair in both cases. The problem is that a module from the standard lib uses a certain value to indicate an illegal index, that has a very legal value in python in general. > I suppose I can't deny that people do things like that, myself included, It is not about what people do. If this was about someone implementing find himself and using -1 as an illegal index, I would certainly agree that it was inadvisable to do so. Yet when this is what python with its libary offers the programmer, you seem reluctant find fault with it. > but mixing data sets where -1 is variously an error flag and a valid > index is only going to lead to trouble when the combined data is used. Yet this is what python does. Using -1 variously as an error flag and a valid index and when people complain about that, you say it sounds like whining. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
"Terry Reedy" <[EMAIL PROTECTED]> writes: > The fact that the -1 return *has* lead to bugs in actual code is the > primary reason Guido has currently decided that find and rfind should go. > A careful review of current usages in the standard library revealed at > least a couple bugs even there. Really it's x[-1]'s behavior that should go, not find/rfind. Will socket.connect_ex also go? How about dict.get? Being able to return some reasonable value for "failure" is a good thing, if failure is expected. Exceptions are for unexpected, i.e., exceptional failures. -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
"Steve Holden" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > Antoon Pardon wrote: >> So what happens if you have a module that is collecting string-index >> pair, colleted from various other parts. In one part you >> want to select the last letter, so you pythonically choose -1 as >> index. In an other part you get a result of find and are happy >> with -1 as an indictation for an invalid index. Then these >> data meet. >> > That's when debugging has to start. Mixing data of such types is > somewhat inadvisable, don't you agree? > > I suppose I can't deny that people do things like that, myself included, > but mixing data sets where -1 is variously an error flag and a valid > index is only going to lead to trouble when the combined data is used. The fact that the -1 return *has* lead to bugs in actual code is the primary reason Guido has currently decided that find and rfind should go. A careful review of current usages in the standard library revealed at least a couple bugs even there. Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Antoon Pardon wrote: > Op 2005-08-27, Steve Holden schreef <[EMAIL PROTECTED]>: > >>> >>If you want an exception from your code when 'w' isn't in the string you >>should consider using index() rather than find. > > > Sometimes it is convenient to have the exception thrown at a later > time. > > >>Otherwise, whatever find() returns you will have to have an "if" in >>there to handle the not-found case. > > > And maybe the more convenient place for this "if" is in a whole different > part of your program, a part where using -1 as an invalid index isn't > at all obvious. > > >>This just sounds like whining to me. If you want to catch errors, use a >>function that will raise an exception rather than relying on the >>invalidity of the result. > > > You always seem to look at such things in a very narrow scope. You never > seem to consider that various parts of a program have to work together. > Or perhaps it's just that I try not to mix parts inappropriately. > So what happens if you have a module that is collecting string-index > pair, colleted from various other parts. In one part you > want to select the last letter, so you pythonically choose -1 as > index. In an other part you get a result of find and are happy > with -1 as an indictation for an invalid index. Then these > data meet. > That's when debugging has to start. Mixing data of such types is somewhat inadvisable, don't you agree? I suppose I can't deny that people do things like that, myself included, but mixing data sets where -1 is variously an error flag and a valid index is only going to lead to trouble when the combined data is used. regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Antoon Pardon wrote: > I think a properly implented find is better than an index. See the current thread in python-dev[1], which proposes a new method, str.partition(). I believe that Raymond Hettinger has shown that almost all uses of str.find() can be more clearly be represented with his proposed function. STeVe [1]http://mail.python.org/pipermail/python-dev/2005-August/055781.html -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Magnus Lycka wrote: > Robert Kern wrote: > >>If I may digress for a bit, my advisor is currently working on a project >>that is processing seafloor depth datasets starting from a few decades >>ago. A lot of this data was orginally to be processed using FORTRAN >>software, so in the idiom of much FORTRAN software from those days, >>is often used to mark missing data. Unfortunately, is a perfectly >>valid datum in most of the unit systems used by the various datasets. >> >>Now he has to find a grad student to traul through the datasets and >>clean up the really invalid 's (as well as other such fun tasks like >>deciding if a dataset that says it's using feet is actually using meters). > > I'm afraid this didn't end with FORTRAN. It's not that long ago > that I wrote a program for my wife that combined a data editor > with a graph display, so that she could clean up time lines with > length and weight data for children (from an international research > project performed during the 90's). 99cm is not unreasonable as a > length, but if you see it in a graph with other length measurements, > it's easy to spot most of the false ones, just as mistyped year part > in a date (common in the beginning of a new year). > > Perhaps graphics can help this grad student too? It's certainly much > easier to spot deviations in curves than in an endless line of > numbers if the curves would normally be reasonably smooth. Yes! In fact, that was the context of the discussion when my advisor told me about this project. Another student had written an interactive GUI for exploring bathymetry maps. My advisor: "That kind of thing would be really great for this new project, etc. etc." -- Robert Kern [EMAIL PROTECTED] "In the fields of hell where the grass grows high Are the graves of dreams allowed to die." -- Richard Harter -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Op 2005-08-27, Steve Holden schreef <[EMAIL PROTECTED]>: >> >> > If you want an exception from your code when 'w' isn't in the string you > should consider using index() rather than find. Sometimes it is convenient to have the exception thrown at a later time. > Otherwise, whatever find() returns you will have to have an "if" in > there to handle the not-found case. And maybe the more convenient place for this "if" is in a whole different part of your program, a part where using -1 as an invalid index isn't at all obvious. > This just sounds like whining to me. If you want to catch errors, use a > function that will raise an exception rather than relying on the > invalidity of the result. You always seem to look at such things in a very narrow scope. You never seem to consider that various parts of a program have to work together. So what happens if you have a module that is collecting string-index pair, colleted from various other parts. In one part you want to select the last letter, so you pythonically choose -1 as index. In an other part you get a result of find and are happy with -1 as an indictation for an invalid index. Then these data meet. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Op 2005-08-27, Terry Reedy schreef <[EMAIL PROTECTED]>: > > "Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message > news:[EMAIL PROTECTED] >> "Terry Reedy" <[EMAIL PROTECTED]> writes: >>> The try/except pattern is a pretty basic part of Python's design. One >>> could say the same about clutter for *every* function or method that >>> raises >>> an exception on invalid input. Should more or even all be duplicated? >>> Why >>> just this one? >> >> Someone must have thought str.find was worth having, or else it >> wouldn't be in the library. > > Well, Guido no longer thinks it worth having and emphatically agreed that > it should be added to one of the 'To be removed' sections of PEP 3000. I think a properly implented find is better than an index. If we only have index, Then asking for permission is no longer a possibility. If we have a find that returns None, we can either ask permission before we index or be forgiven by the exception that is raised. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Robert Kern wrote: > If I may digress for a bit, my advisor is currently working on a project > that is processing seafloor depth datasets starting from a few decades > ago. A lot of this data was orginally to be processed using FORTRAN > software, so in the idiom of much FORTRAN software from those days, > is often used to mark missing data. Unfortunately, is a perfectly > valid datum in most of the unit systems used by the various datasets. > > Now he has to find a grad student to traul through the datasets and > clean up the really invalid 's (as well as other such fun tasks like > deciding if a dataset that says it's using feet is actually using meters). I'm afraid this didn't end with FORTRAN. It's not that long ago that I wrote a program for my wife that combined a data editor with a graph display, so that she could clean up time lines with length and weight data for children (from an international research project performed during the 90's). 99cm is not unreasonable as a length, but if you see it in a graph with other length measurements, it's easy to spot most of the false ones, just as mistyped year part in a date (common in the beginning of a new year). Perhaps graphics can help this grad student too? It's certainly much easier to spot deviations in curves than in an endless line of numbers if the curves would normally be reasonably smooth. -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Bryan Olson wrote: > Steve Holden wrote: > > Paul Rubin wrote: > > We are arguing about trivialities here. Let's stop before it gets > > interesting :-) > > Some of us are looking beyond the trivia of what string.find() > should return, at an unfortunate interaction of Python features, > brought on by the special-casing of negative indexes. The wart > bites novice or imperfect Python programmers in simple cases > such as string.find(), or when their subscripts accidentally > fall off the low end. It bites programmers who want to fully > implement Python slicing, because of the double-and- > contradictory- interpretation of -1, as both an exclusive ending > bound and the index of the last element. It bites documentation > authors who naturally think of the non-negative subscript as > *the* index of a sequence item. > > Sure. I wrote two days ago: > We might agree, before further discussion, that this isn't the most > elegant part of Python's design, and it's down to history that this tiny > little wart remains. While I agree it's a trap for the unwary I still don't regard it as a major wart. But I'm all in favor of discussions to make 3.0 a better language. regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find
I agree with Bryan Olson. I think it's a kind of bug, and it has to be fixed, like few other things. But I understand that this change can give little problems to the already written code... Bye, bearophile -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Steve Holden wrote: > Paul Rubin wrote: > We are arguing about trivialities here. Let's stop before it gets > interesting :-) Some of us are looking beyond the trivia of what string.find() should return, at an unfortunate interaction of Python features, brought on by the special-casing of negative indexes. The wart bites novice or imperfect Python programmers in simple cases such as string.find(), or when their subscripts accidentally fall off the low end. It bites programmers who want to fully implement Python slicing, because of the double-and- contradictory- interpretation of -1, as both an exclusive ending bound and the index of the last element. It bites documentation authors who naturally think of the non-negative subscript as *the* index of a sequence item. -- --Bryan -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Paul Rubin wrote: > Steve Holden <[EMAIL PROTECTED]> writes: > >>A corrected find() that returns None on failure is a five-liner. > > > If I wanted to write five lines instead of one everywhere in a Python > program, I'd use Java. We are arguing about trivialities here. Let's stop before it gets interesting :-) regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Paul> Steve Holden <[EMAIL PROTECTED]> writes: >> A corrected find() that returns None on failure is a five-liner. Paul> If I wanted to write five lines instead of one everywhere in a Paul> Python program, I'd use Java. +1 for QOTW. Skip -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Steve Holden <[EMAIL PROTECTED]> writes: > A corrected find() that returns None on failure is a five-liner. If I wanted to write five lines instead of one everywhere in a Python program, I'd use Java. -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Terry Reedy wrote: > "Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message > news:[EMAIL PROTECTED] > >>Steve Holden <[EMAIL PROTECTED]> writes: >> >>>Of course. But onc you (sensibly) decide to use an "if" then there >>>really isn't much difference between -1, None, () and sys.maxint as >>>a sentinel value, is there? >> >>Of course there is. -1 is (under Python's perverse semantics) a valid >>subscript. sys.maxint is an artifact of Python's fixed-size int >>datatype, which is fading away under int/long unification, so it's >>something that soon won't exist and shouldn't be used. None and () >>are invalid subscripts so would be reasonable return values, unlike -1 >>and sys.maxint. Of those, None is preferable to () because of its >>semantic connotations. > > > I agree here that None is importantly different from -1 for the reason > stated. The use of -1 is, I am sure, a holdover from statically typed > languages (C, in particular) that require all return values to be of the > same type, even if the 'return value' is actually meant to indicat that > there is no valid return value. While I agree that it would have been more sensible to choose None in find()'s original design, there's really no reason to go breaking existing code just to fix it. Guido has already agreed that find() can change (or even disappear) in Python 3.0, so please let's just leave things as they are for now. A corrected find() that returns None on failure is a five-liner. regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Paul Rubin wrote: > Steve Holden <[EMAIL PROTECTED]> writes: > >>If you want an exception from your code when 'w' isn't in the string >>you should consider using index() rather than find. > > > The idea is you expect w to be in the string. If w isn't in the > string, your code has a bug, and programs with bugs should fail as > early as possible so you can locate the bugs quickly and easily. That > is why, for example, > > x = 'buggy'[None] > > raises an exception instead of doing something stupid like returning 'g'. You did read the sentence you were replying to, didn't you? regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Steve Holden wrote:
> Bryan Olson wrote:
>> [...] I see no good reason for the following
>> to happily print 'y'.
>>
>> s = 'buggy'
>> print s[s.find('w')]
>>
>> > Before using the result you always have to perform
>> > a test to discriminate between the found and not found cases. So I
>> don't
>> > really see why this wart has put such a bug up your ass.
>>
>> The bug that got me was what a slice object reports as the
>> 'stop' bound when the step is negative and the slice includes
>> index 0. Took me hours to figure out why my code was failing.
>>
>> The double-meaning of -1, as both an exclusive stopping bound
>> and an alias for the highest valid index, is just plain whacked.
>> Unfortunately, as negative indexes are currently handled, there
>> is no it-just-works value that slice could return.
>>
>>
> If you want an exception from your code when 'w' isn't in the string you
> should consider using index() rather than find.
That misses the point. The code is a hypothetical example of
what a novice or imperfect Pythoners might have to deal with.
The exception isn't really wanted; it's just vastly superior to
silently returning a nonsensical value.
> Otherwise, whatever find() returns you will have to have an "if" in
> there to handle the not-found case.
>
> This just sounds like whining to me. If you want to catch errors, use a
> function that will raise an exception rather than relying on the
> invalidity of the result.
I suppose if you ignore the real problems and the proposed
solution, it might sound a lot like whining.
--
--Bryan
--
http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
"Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message news:[EMAIL PROTECTED] > Steve Holden <[EMAIL PROTECTED]> writes: >> Of course. But onc you (sensibly) decide to use an "if" then there >> really isn't much difference between -1, None, () and sys.maxint as >> a sentinel value, is there? > > Of course there is. -1 is (under Python's perverse semantics) a valid > subscript. sys.maxint is an artifact of Python's fixed-size int > datatype, which is fading away under int/long unification, so it's > something that soon won't exist and shouldn't be used. None and () > are invalid subscripts so would be reasonable return values, unlike -1 > and sys.maxint. Of those, None is preferable to () because of its > semantic connotations. I agree here that None is importantly different from -1 for the reason stated. The use of -1 is, I am sure, a holdover from statically typed languages (C, in particular) that require all return values to be of the same type, even if the 'return value' is actually meant to indicat that there is no valid return value. Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Steve Holden <[EMAIL PROTECTED]> writes: > If you want an exception from your code when 'w' isn't in the string > you should consider using index() rather than find. The idea is you expect w to be in the string. If w isn't in the string, your code has a bug, and programs with bugs should fail as early as possible so you can locate the bugs quickly and easily. That is why, for example, x = 'buggy'[None] raises an exception instead of doing something stupid like returning 'g'. -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Steve Holden <[EMAIL PROTECTED]> writes: > Of course. But onc you (sensibly) decide to use an "if" then there > really isn't much difference between -1, None, () and sys.maxint as > a sentinel value, is there? Of course there is. -1 is (under Python's perverse semantics) a valid subscript. sys.maxint is an artifact of Python's fixed-size int datatype, which is fading away under int/long unification, so it's something that soon won't exist and shouldn't be used. None and () are invalid subscripts so would be reasonable return values, unlike -1 and sys.maxint. Of those, None is preferable to () because of its semantic connotations. -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Steve Holden wrote: > Of course. But onc you (sensibly) decide to use an "if" then there > really isn't much difference between -1, None, () and sys.maxint as > a sentinel value, is there? Sure there is. -1 is a valid index; None is not. -1 as a sentinel is specific to str.find(); None is used all over Python as a sentinel. If I may digress for a bit, my advisor is currently working on a project that is processing seafloor depth datasets starting from a few decades ago. A lot of this data was orginally to be processed using FORTRAN software, so in the idiom of much FORTRAN software from those days, is often used to mark missing data. Unfortunately, is a perfectly valid datum in most of the unit systems used by the various datasets. Now he has to find a grad student to traul through the datasets and clean up the really invalid 's (as well as other such fun tasks like deciding if a dataset that says it's using feet is actually using meters). I have already called "Not It." -- Robert Kern [EMAIL PROTECTED] "In the fields of hell where the grass grows high Are the graves of dreams allowed to die." -- Richard Harter -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Bryan Olson wrote:
> Steve Holden wrote:
> > Bryan Olson wrote:
> >> Antoon Pardon wrote:
>
> >> > It probably is too late now, but I always felt, find should
> >> > have returned None when the substring isn't found.
> >>
> >> None is certainly a reasonable candidate.
> [...]
> >> The really broken part is that unsuccessful searches return a
> >> legal index.
> >>
> > We might agree, before further discussion, that this isn't the most
> > elegant part of Python's design, and it's down to history that this tiny
> > little wart remains.
>
> I don't think my proposal breaks historic Python code, and I
> don't think it has the same kind of unfortunate subtle
> consequences as the current indexing scheme. You may think the
> wart is tiny, but the duct-tape* is available so let's cure it.
>
> [*] http://www.google.com/search?as_q=warts+%22duct+tape%22
>
>
> >> My suggestion doesn't change what find() returns, and doesn't
> >> break code. Negative one is a reasonable choice to represent an
> >> unsuccessful search -- provided it is not a legal index. Instead
> >> of changing what find() returns, we should heal the
> >> special-case-when-index-is-negative-in-a-certain-range wart.
> >>
> >>
> > What I don't understand is why you want it to return something that
> > isn't a legal index.
>
> In this case, so that errors are caught as close to their
> occurrence as possible. I see no good reason for the following
> to happily print 'y'.
>
> s = 'buggy'
> print s[s.find('w')]
>
> > Before using the result you always have to perform
> > a test to discriminate between the found and not found cases. So I don't
> > really see why this wart has put such a bug up your ass.
>
> The bug that got me was what a slice object reports as the
> 'stop' bound when the step is negative and the slice includes
> index 0. Took me hours to figure out why my code was failing.
>
> The double-meaning of -1, as both an exclusive stopping bound
> and an alias for the highest valid index, is just plain whacked.
> Unfortunately, as negative indexes are currently handled, there
> is no it-just-works value that slice could return.
>
>
If you want an exception from your code when 'w' isn't in the string you
should consider using index() rather than find.
Otherwise, whatever find() returns you will have to have an "if" in
there to handle the not-found case.
This just sounds like whining to me. If you want to catch errors, use a
function that will raise an exception rather than relying on the
invalidity of the result.
regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC http://www.holdenweb.com/
--
http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Torsten Bronger wrote: > Hallöchen! > > "Terry Reedy" <[EMAIL PROTECTED]> writes: > > >>"Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message >>news:[EMAIL PROTECTED] >> >> >>>"Terry Reedy" <[EMAIL PROTECTED]> writes: >>> >>> Str.find is redundant with the Pythonic exception-raising str.index and I think it should be removed in Py3. >>> >>>I like having it available so you don't have to clutter your code >>>with try/except if the substring isn't there. But it should not >>>return a valid integer index. >> >>The try/except pattern is a pretty basic part of Python's design. >>One could say the same about clutter for *every* function or >>method that raises an exception on invalid input. Should more or >>even all be duplicated? Why just this one? > > > Granted, try/except can be used for deliberate case discrimination > (which may even happen in the standard library in many places), > however, it is only the second most elegant method -- the most > elegant being "if". Where "if" does the job, it should be prefered > in my opinion. > Of course. But onc you (sensibly) decide to use an "if" then there really isn't much difference between -1, None, () and sys.maxint as a sentinel value, is there? Which is what I've been trying to say all along. regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
"Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message news:[EMAIL PROTECTED] > "Terry Reedy" <[EMAIL PROTECTED]> writes: >> The try/except pattern is a pretty basic part of Python's design. One >> could say the same about clutter for *every* function or method that >> raises >> an exception on invalid input. Should more or even all be duplicated? >> Why >> just this one? > > Someone must have thought str.find was worth having, or else it > wouldn't be in the library. Well, Guido no longer thinks it worth having and emphatically agreed that it should be added to one of the 'To be removed' sections of PEP 3000. Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
"Terry Reedy" <[EMAIL PROTECTED]> writes: > The try/except pattern is a pretty basic part of Python's design. One > could say the same about clutter for *every* function or method that raises > an exception on invalid input. Should more or even all be duplicated? Why > just this one? Someone must have thought str.find was worth having, or else it wouldn't be in the library. -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
Hallöchen! "Terry Reedy" <[EMAIL PROTECTED]> writes: > "Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message > news:[EMAIL PROTECTED] > >> "Terry Reedy" <[EMAIL PROTECTED]> writes: >> >>> Str.find is redundant with the Pythonic exception-raising >>> str.index and I think it should be removed in Py3. >> >> I like having it available so you don't have to clutter your code >> with try/except if the substring isn't there. But it should not >> return a valid integer index. > > The try/except pattern is a pretty basic part of Python's design. > One could say the same about clutter for *every* function or > method that raises an exception on invalid input. Should more or > even all be duplicated? Why just this one? Granted, try/except can be used for deliberate case discrimination (which may even happen in the standard library in many places), however, it is only the second most elegant method -- the most elegant being "if". Where "if" does the job, it should be prefered in my opinion. Tschö, Torsten. -- Torsten Bronger, aquisgrana, europa vetusICQ 264-296-646 -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
"Paul Rubin" <"http://phr.cx"@NOSPAM.invalid> wrote in message news:[EMAIL PROTECTED] > "Terry Reedy" <[EMAIL PROTECTED]> writes: >>Str.find is >> redundant with the Pythonic exception-raising str.index >> and I think it should be removed in Py3. > > I like having it available so you don't have to clutter your code with > try/except if the substring isn't there. But it should not return a > valid integer index. The try/except pattern is a pretty basic part of Python's design. One could say the same about clutter for *every* function or method that raises an exception on invalid input. Should more or even all be duplicated? Why just this one? Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
"Terry Reedy" <[EMAIL PROTECTED]> writes: > I agree in this sense: the use of any int as an error return is an > unPythonic *nix-Cism, which I believe was copied therefrom. Str.find is > redundant with the Pythonic exception-raising str.index and I think it > should be removed in Py3. I like having it available so you don't have to clutter your code with try/except if the substring isn't there. But it should not return a valid integer index. -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
"Bryan Olson" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > The double-meaning of -1, as both an exclusive stopping bound > and an alias for the highest valid index, is just plain whacked. I agree in this sense: the use of any int as an error return is an unPythonic *nix-Cism, which I believe was copied therefrom. Str.find is redundant with the Pythonic exception-raising str.index and I think it should be removed in Py3. Therefore, I think changing it now is untimely and changing the language because of it backwards. Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Bryan Olson wrote: > Steve Holden wrote: > > Bryan Olson wrote: > >> Antoon Pardon wrote: > > >> > It probably is too late now, but I always felt, find should > >> > have returned None when the substring isn't found. > >> > >> None is certainly a reasonable candidate. > [...] > >> The really broken part is that unsuccessful searches return a > >> legal index. > >> > > We might agree, before further discussion, that this isn't the most > > elegant part of Python's design, and it's down to history that this tiny > > little wart remains. > > I don't think my proposal breaks historic Python code, and I > don't think it has the same kind of unfortunate subtle > consequences as the current indexing scheme. You may think the > wart is tiny, but the duct-tape* is available so let's cure it. > > [*] http://www.google.com/search?as_q=warts+%22duct+tape%22 Well, nobody stops you from posting this on python-dev and be screamed at by Guido... just-kidding-ly Reinhold -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Steve Holden wrote:
> Bryan Olson wrote:
>> Antoon Pardon wrote:
>> > It probably is too late now, but I always felt, find should
>> > have returned None when the substring isn't found.
>>
>> None is certainly a reasonable candidate.
[...]
>> The really broken part is that unsuccessful searches return a
>> legal index.
>>
> We might agree, before further discussion, that this isn't the most
> elegant part of Python's design, and it's down to history that this tiny
> little wart remains.
I don't think my proposal breaks historic Python code, and I
don't think it has the same kind of unfortunate subtle
consequences as the current indexing scheme. You may think the
wart is tiny, but the duct-tape* is available so let's cure it.
[*] http://www.google.com/search?as_q=warts+%22duct+tape%22
>> My suggestion doesn't change what find() returns, and doesn't
>> break code. Negative one is a reasonable choice to represent an
>> unsuccessful search -- provided it is not a legal index. Instead
>> of changing what find() returns, we should heal the
>> special-case-when-index-is-negative-in-a-certain-range wart.
>>
>>
> What I don't understand is why you want it to return something that
> isn't a legal index.
In this case, so that errors are caught as close to their
occurrence as possible. I see no good reason for the following
to happily print 'y'.
s = 'buggy'
print s[s.find('w')]
> Before using the result you always have to perform
> a test to discriminate between the found and not found cases. So I don't
> really see why this wart has put such a bug up your ass.
The bug that got me was what a slice object reports as the
'stop' bound when the step is negative and the slice includes
index 0. Took me hours to figure out why my code was failing.
The double-meaning of -1, as both an exclusive stopping bound
and an alias for the highest valid index, is just plain whacked.
Unfortunately, as negative indexes are currently handled, there
is no it-just-works value that slice could return.
--
--Bryan
--
http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Bryan Olson wrote: > Antoon Pardon wrote: > > Bryan Olson schreef: > > > >>Steve Holden asked: > >>>And what are you proposing that > >>>find() should return if the substring isn't found at all? please don't > >>>suggest it should raise an exception, as index() exists to provide that > >>>functionality. > >> > >>There are a number of good options. A legal index is not one of > >>them. > > > > IMO, with find a number of "features" of python come together. > > that create an awkward situation. > > > > 1) 0 is a false value, but indexes start at 0 so you can't > >return 0 to indicate nothing was found. > > > > 2) -1 is returned, which is both a true value and a legal > >index. > > > > It probably is too late now, but I always felt, find should > > have returned None when the substring isn't found. > > None is certainly a reasonable candidate. The one-past-the-end > value, len(sequence), would be fine, and follows the preferred > idiom of C/C++. I don't see any elegant way to arrange for > successful finds always to return a true value and unsuccessful > calls to return a false value. > > The really broken part is that unsuccessful searches return a > legal index. > We might agree, before further discussion, that this isn't the most elegant part of Python's design, and it's down to history that this tiny little wart remains. > My suggestion doesn't change what find() returns, and doesn't > break code. Negative one is a reasonable choice to represent an > unsuccessful search -- provided it is not a legal index. Instead > of changing what find() returns, we should heal the > special-case-when-index-is-negative-in-a-certain-range wart. > > What I don't understand is why you want it to return something that isn't a legal index. Before using the result you always have to perform a test to discriminate between the found and not found cases. So I don't really see why this wart has put such a bug up your ass. regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Bryan Olson <[EMAIL PROTECTED]> wrote in news:[EMAIL PROTECTED]: > Steve Holden asked: > > Do you just go round looking for trouble? > > In the course of programming, yes, absolutly. > > > As far as position reporting goes, it seems pretty clear that > > find() will always report positive index values. In a > > five-character string then -1 and 4 are effectively > > equivalent. > > > > What on earth makes you call this a bug? > > What you just said, versus what the doc says. > > > And what are you proposing that > > find() should return if the substring isn't found at all? > > please don't suggest it should raise an exception, as index() > > exists to provide that functionality. > > There are a number of good options. A legal index is not one of > them. > > Practically speaking, what difference would it make? Supposing find returned None for not-found. How would you use it in your code that would make it superior to what happens now? In either case you would have to test for the not-found state before relying on the index returned, wouldn't you? Or do you have a use that would eliminate that step? -- rzed -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Antoon Pardon wrote: > Bryan Olson schreef: > >>Steve Holden asked: >>>And what are you proposing that >>>find() should return if the substring isn't found at all? please don't >>>suggest it should raise an exception, as index() exists to provide that >>>functionality. >> >>There are a number of good options. A legal index is not one of >>them. > > IMO, with find a number of "features" of python come together. > that create an awkward situation. > > 1) 0 is a false value, but indexes start at 0 so you can't >return 0 to indicate nothing was found. > > 2) -1 is returned, which is both a true value and a legal >index. > > It probably is too late now, but I always felt, find should > have returned None when the substring isn't found. None is certainly a reasonable candidate. The one-past-the-end value, len(sequence), would be fine, and follows the preferred idiom of C/C++. I don't see any elegant way to arrange for successful finds always to return a true value and unsuccessful calls to return a false value. The really broken part is that unsuccessful searches return a legal index. My suggestion doesn't change what find() returns, and doesn't break code. Negative one is a reasonable choice to represent an unsuccessful search -- provided it is not a legal index. Instead of changing what find() returns, we should heal the special-case-when-index-is-negative-in-a-certain-range wart. -- --Bryan -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Op 2005-08-25, Bryan Olson schreef <[EMAIL PROTECTED]>: > Steve Holden asked: > > Do you just go round looking for trouble? > > In the course of programming, yes, absolutly. > > > As far as position reporting goes, it seems pretty clear that find() > > will always report positive index values. In a five-character string > > then -1 and 4 are effectively equivalent. > > > > What on earth makes you call this a bug? > > What you just said, versus what the doc says. > > > And what are you proposing that > > find() should return if the substring isn't found at all? please don't > > suggest it should raise an exception, as index() exists to provide that > > functionality. > > There are a number of good options. A legal index is not one of > them. IMO, with find a number of "features" of python come together. that create an awkward situation. 1) 0 is a false value, but indexes start at 0 so you can't return 0 to indicate nothing was found. 2) -1 is returned, which is both a true value and a legal index. It probably is too late now, but I always felt, find should have returned None when the substring isn't found. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Steve Holden asked: > Do you just go round looking for trouble? In the course of programming, yes, absolutly. > As far as position reporting goes, it seems pretty clear that find() > will always report positive index values. In a five-character string > then -1 and 4 are effectively equivalent. > > What on earth makes you call this a bug? What you just said, versus what the doc says. > And what are you proposing that > find() should return if the substring isn't found at all? please don't > suggest it should raise an exception, as index() exists to provide that > functionality. There are a number of good options. A legal index is not one of them. -- --Bryan -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Steve Holden <[EMAIL PROTECTED]> writes: > As far as position reporting goes, it seems pretty clear that find() > will always report positive index values. In a five-character string > then -1 and 4 are effectively equivalent. > > What on earth makes you call this a bug? And what are you proposing > that find() should return if the substring isn't found at all? please > don't suggest it should raise an exception, as index() exists to > provide that functionality. Bryan is making the case that Python's use of negative subscripts to measure from the end of sequences is bogus, and that it should be done some other way instead. I've certainly had bugs in my own programs related to that "feature". -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing,was Re: Bug in slice type
On Thu, 25 Aug 2005 00:05:18 -0400
Steve Holden wrote:
> What on earth makes you call this a bug? And what are you proposing that
> find() should return if the substring isn't found at all? please don't
> suggest it should raise an exception, as index() exists to provide that
> functionality.
Returning -1 looks like C-ism for me. It could better return None when none
is found.
index = "Hello".find("z")
if index is not None:
# ...
Now it's too late for it, I know.
--
jk
--
http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
>contained in the range [start, end) Does range(start, end) generate negative integers in Python if start >= 0 and end >= start? -- Regards, Casey -- http://mail.python.org/mailman/listinfo/python-list
Re: Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type
Bryan Olson wrote:
> The doc for the find() method of string objects, which is
> essentially the same as the string.find() function, states:
>
> find(sub[, start[, end]])
>Return the lowest index in the string where substring sub
>is found, such that sub is contained in the range [start,
>end). Optional arguments start and end are interpreted as
>in slice notation. Return -1 if sub is not found.
>
> Consider:
>
> print 'Hello'.find('o')
>
> or:
>
> import string
> print string.find('Hello', 'o')
>
> The substring 'o' is found in 'Hello' at the index -1, and at
> the index 4, and it is not found at any other index. Both the
> locations found are in the range [start, end), and obviously -1
> is less than 4, so according to the documentation, find() should
> return -1.
>
> What the either of the above actually prints is:
>
> 4
>
> which shows yet another bug resulting from Python's handling of
> negative indexes. This one is clearly a documentation error, but
> the real fix is to cure the wart so that Python's behavior is
> consistent enough that we'll be able to describe it correctly.
>
>
Do you just go round looking for trouble?
As far as position reporting goes, it seems pretty clear that find()
will always report positive index values. In a five-character string
then -1 and 4 are effectively equivalent.
What on earth makes you call this a bug? And what are you proposing that
find() should return if the substring isn't found at all? please don't
suggest it should raise an exception, as index() exists to provide that
functionality.
regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC http://www.holdenweb.com/
--
http://mail.python.org/mailman/listinfo/python-list
