Re: Checking if elements are empty
Neil Cerutti wrote: > On 2007-09-10, Steve Holden <[EMAIL PROTECTED]> wrote: >>> I have a quibble not with the functionality of the boolean check, >>> but with its expressiveness. if y[0] == "" expresses more, i.e., >>> that I expect y[0] to contain a Python byte string. >> I have a quibble with a test that will raise an exception when >> the anticipated condition is true. Your test is patently >> absurd, as you would have discovered had you bothered to try >> it: > > "if y[0] == "" expresses more, i.e., that I expect y[0] to > contain a Python byte string." > That isn't unreasonable immediately after y = "...".split(...) though, is it? I was talking tosh above. regards Steve -- Steve Holden+1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden --- Asciimercial -- Get on the web: Blog, lens and tag the Internet Many services currently offer free registration --- Thank You for Reading - -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
Hamilton, William wrote:
>> From: Steve Holden
>> Neil Cerutti wrote:
>>> On 2007-09-10, Chris Mellon <[EMAIL PROTECTED]> wrote:
On 9/10/07, Neil Cerutti <[EMAIL PROTECTED]> wrote:
> Agreed; but I prefer 'if y[0] == ""', absent more context and
> better names.
Probably should use u"" if you're going to take that route, as
this will fail spuriously if y[0] contains a unicode string
that can't be implicitly converted to ascii. Personally, I
prefer the boolean check and I'll let operations fail elsewhere
if there's a type mismatch.
>>> I have a quibble not with the functionality of the boolean check,
>>> but with its expressiveness. if y[0] == "" expresses more, i.e.,
>>> that I expect y[0] to contain a Python byte string.
>>>
>> I have a quibble with a test that will raise an exception when the
>> anticipated condition is true. Your test is patently absurd, as you
>> would have discovered had you bothered to try it:
>>
>> >>> y = ""
>> >>> if y[0] == "":
>> ... print "True"
>> ... else:
>> ... print "False"
>> ...
>> Traceback (most recent call last):
>>File "", line 1, in
>> IndexError: string index out of range
>> >>>
>>
>> Further, when the string is *not* the null string the test will always
>> return False, as you will be comparing two strings of unequal length.
>>
>> So, absent a solution that works, len(y) == 0 looks pretty good.
>
>
> Going back to the OP, the problem is taking a string such as
x = ' \t"ff'
> then splitting that string like this
y = x.split('\t')
>
> The question is, does the first element of the list y contain an empty
> string or not? In this case, the logic in the following conditional is
> perfectly valid.
if y[0] == "":
> ...print "True"
> ... else
> ...print "False"
>
> (len(y[0]) == 0) would also work, and is the solution you originally
> gave the OP.
>
Aah, list of strings, right. Thanks.
regards
Steve
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RE: Checking if elements are empty
> From: Steve Holden
> Neil Cerutti wrote:
> > On 2007-09-10, Chris Mellon <[EMAIL PROTECTED]> wrote:
> >> On 9/10/07, Neil Cerutti <[EMAIL PROTECTED]> wrote:
> >>> Agreed; but I prefer 'if y[0] == ""', absent more context and
> >>> better names.
> >> Probably should use u"" if you're going to take that route, as
> >> this will fail spuriously if y[0] contains a unicode string
> >> that can't be implicitly converted to ascii. Personally, I
> >> prefer the boolean check and I'll let operations fail elsewhere
> >> if there's a type mismatch.
> >
> > I have a quibble not with the functionality of the boolean check,
> > but with its expressiveness. if y[0] == "" expresses more, i.e.,
> > that I expect y[0] to contain a Python byte string.
> >
> I have a quibble with a test that will raise an exception when the
> anticipated condition is true. Your test is patently absurd, as you
> would have discovered had you bothered to try it:
>
> >>> y = ""
> >>> if y[0] == "":
> ... print "True"
> ... else:
> ... print "False"
> ...
> Traceback (most recent call last):
>File "", line 1, in
> IndexError: string index out of range
> >>>
>
> Further, when the string is *not* the null string the test will always
> return False, as you will be comparing two strings of unequal length.
>
> So, absent a solution that works, len(y) == 0 looks pretty good.
Going back to the OP, the problem is taking a string such as
>>> x = ' \t"ff'
then splitting that string like this
>>> y = x.split('\t')
The question is, does the first element of the list y contain an empty
string or not? In this case, the logic in the following conditional is
perfectly valid.
>>> if y[0] == "":
...print "True"
... else
...print "False"
(len(y[0]) == 0) would also work, and is the solution you originally
gave the OP.
--
-Bill Hamilton
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Re: Checking if elements are empty
On 2007-09-10, Steve Holden <[EMAIL PROTECTED]> wrote: >> I have a quibble not with the functionality of the boolean check, >> but with its expressiveness. if y[0] == "" expresses more, i.e., >> that I expect y[0] to contain a Python byte string. > > I have a quibble with a test that will raise an exception when > the anticipated condition is true. Your test is patently > absurd, as you would have discovered had you bothered to try > it: "if y[0] == "" expresses more, i.e., that I expect y[0] to contain a Python byte string." -- Neil Cerutti Bach's death is attributed to the end of the Baroque era. --Music Lit Essay -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
Steve Holden wrote: > Neil Cerutti wrote: > >>> y = "" > >>> if y[0] == "": > ... print "True" > ... else: > ... print "False" > ... > Traceback (most recent call last): > File "", line 1, in > IndexError: string index out of range > >>> > Uhm, weren't we talking about a list of strings? /W -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
> "Chris Mellon" <[EMAIL PROTECTED]> (CM) wrote: >CM> On 9/10/07, Neil Cerutti <[EMAIL PROTECTED]> wrote: >>> Agreed; but I prefer 'if y[0] == ""', absent more context and >>> better names. >CM> Probably should use u"" if you're going to take that route, as this >CM> will fail spuriously if y[0] contains a unicode string that can't be >CM> implicitly converted to ascii. Personally, I prefer the boolean check >CM> and I'll let operations fail elsewhere if there's a type mismatch. >>> x=u"€" >>> x == "" False >>> -- Piet van Oostrum <[EMAIL PROTECTED]> URL: http://www.cs.uu.nl/~piet [PGP 8DAE142BE17999C4] Private email: [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
Neil Cerutti wrote: > On 2007-09-10, Chris Mellon <[EMAIL PROTECTED]> wrote: >> On 9/10/07, Neil Cerutti <[EMAIL PROTECTED]> wrote: >>> Agreed; but I prefer 'if y[0] == ""', absent more context and >>> better names. >> Probably should use u"" if you're going to take that route, as >> this will fail spuriously if y[0] contains a unicode string >> that can't be implicitly converted to ascii. Personally, I >> prefer the boolean check and I'll let operations fail elsewhere >> if there's a type mismatch. > > I have a quibble not with the functionality of the boolean check, > but with its expressiveness. if y[0] == "" expresses more, i.e., > that I expect y[0] to contain a Python byte string. > I have a quibble with a test that will raise an exception when the anticipated condition is true. Your test is patently absurd, as you would have discovered had you bothered to try it: >>> y = "" >>> if y[0] == "": ... print "True" ... else: ... print "False" ... Traceback (most recent call last): File "", line 1, in IndexError: string index out of range >>> Further, when the string is *not* the null string the test will always return False, as you will be comparing two strings of unequal length. So, absent a solution that works, len(y) == 0 looks pretty good. regards Steve -- Steve Holden+1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden --- Asciimercial -- Get on the web: Blog, lens and tag the Internet Many services currently offer free registration --- Thank You for Reading - -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
On 2007-09-10, Chris Mellon <[EMAIL PROTECTED]> wrote: > On 9/10/07, Neil Cerutti <[EMAIL PROTECTED]> wrote: >> Agreed; but I prefer 'if y[0] == ""', absent more context and >> better names. > > Probably should use u"" if you're going to take that route, as > this will fail spuriously if y[0] contains a unicode string > that can't be implicitly converted to ascii. Personally, I > prefer the boolean check and I'll let operations fail elsewhere > if there's a type mismatch. I have a quibble not with the functionality of the boolean check, but with its expressiveness. if y[0] == "" expresses more, i.e., that I expect y[0] to contain a Python byte string. -- Neil Cerutti -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
On 9/10/07, Neil Cerutti <[EMAIL PROTECTED]> wrote:
> On 2007-09-10, Chris Mellon <[EMAIL PROTECTED]> wrote:
> > On 9/10/07, Neil Cerutti <[EMAIL PROTECTED]> wrote:
> >> On 2007-09-08, Wildemar Wildenburger <[EMAIL PROTECTED]> wrote:
> >> > Lawrence D'Oliveiro wrote:
> >> > if y[0]:
> >> Not a good idea.
> >> >>> Why not?
> >> >>
> >> >> Because there is a situation where your version of the test
> >> >> will fail even if the first element of y is non-null.
> >> >
> >> > Such as? Seriously people, a little more verbosity wouldn't
> >> > hurt here. This isn't a mystery game.
> >>
> >> >>> if "": True
> >> ...
> >> >>> if 0: True
> >> ...
> >> >>> if []: True
> >> ...
> >> >>> if {}: True
> >> ...
> >> >>> if None: True
> >> ...
> >>
> >> That's may not be all the possibilities. Lots of Python objects
> >> can evaluate to false in a boolean context.
> >
> > All of these except 0 and None will also return False if you
> > check the length. In fact, I am not aware of any builtin where
> > len() will return 0, but it will be true in a boolean context.
> >
> > If you're advocating being more specific in the hope of
> > catching errors, this is a pretty terrible example. The OP
> > wasn't checking for "null", whatever that means - he was
> > checking for the empty string, in a list known (or intended) to
> > contain only strings. The simple boolean check is totally
> > appropriate.
>
> Agreed; but I prefer 'if y[0] == ""', absent more context and
> better names.
>
Probably should use u"" if you're going to take that route, as this
will fail spuriously if y[0] contains a unicode string that can't be
implicitly converted to ascii. Personally, I prefer the boolean check
and I'll let operations fail elsewhere if there's a type mismatch.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
On 2007-09-10, Chris Mellon <[EMAIL PROTECTED]> wrote:
> On 9/10/07, Neil Cerutti <[EMAIL PROTECTED]> wrote:
>> On 2007-09-08, Wildemar Wildenburger <[EMAIL PROTECTED]> wrote:
>> > Lawrence D'Oliveiro wrote:
>> > if y[0]:
>> Not a good idea.
>> >>> Why not?
>> >>
>> >> Because there is a situation where your version of the test
>> >> will fail even if the first element of y is non-null.
>> >
>> > Such as? Seriously people, a little more verbosity wouldn't
>> > hurt here. This isn't a mystery game.
>>
>> >>> if "": True
>> ...
>> >>> if 0: True
>> ...
>> >>> if []: True
>> ...
>> >>> if {}: True
>> ...
>> >>> if None: True
>> ...
>>
>> That's may not be all the possibilities. Lots of Python objects
>> can evaluate to false in a boolean context.
>
> All of these except 0 and None will also return False if you
> check the length. In fact, I am not aware of any builtin where
> len() will return 0, but it will be true in a boolean context.
>
> If you're advocating being more specific in the hope of
> catching errors, this is a pretty terrible example. The OP
> wasn't checking for "null", whatever that means - he was
> checking for the empty string, in a list known (or intended) to
> contain only strings. The simple boolean check is totally
> appropriate.
Agreed; but I prefer 'if y[0] == ""', absent more context and
better names.
--
Neil Cerutti
--
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Re: Checking if elements are empty
On 9/10/07, Neil Cerutti <[EMAIL PROTECTED]> wrote:
> On 2007-09-08, Wildemar Wildenburger <[EMAIL PROTECTED]> wrote:
> > Lawrence D'Oliveiro wrote:
> > if y[0]:
> Not a good idea.
> >>> Why not?
> >>
> >> Because there is a situation where your version of the test
> >> will fail even if the first element of y is non-null.
> >
> > Such as? Seriously people, a little more verbosity wouldn't
> > hurt here. This isn't a mystery game.
>
> >>> if "": True
> ...
> >>> if 0: True
> ...
> >>> if []: True
> ...
> >>> if {}: True
> ...
> >>> if None: True
> ...
>
> That's may not be all the possibilities. Lots of Python objects
> can evaluate to false in a boolean context.
>
All of these except 0 and None will also return False if you check the
length. In fact, I am not aware of any builtin where len() will return
0, but it will be true in a boolean context.
If you're advocating being more specific in the hope of catching
errors, this is a pretty terrible example. The OP wasn't checking for
"null", whatever that means - he was checking for the empty string, in
a list known (or intended) to contain only strings. The simple boolean
check is totally appropriate.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
On 2007-09-08, Wildemar Wildenburger <[EMAIL PROTECTED]> wrote:
> Lawrence D'Oliveiro wrote:
> if y[0]:
Not a good idea.
>>> Why not?
>>
>> Because there is a situation where your version of the test
>> will fail even if the first element of y is non-null.
>
> Such as? Seriously people, a little more verbosity wouldn't
> hurt here. This isn't a mystery game.
>>> if "": True
...
>>> if 0: True
...
>>> if []: True
...
>>> if {}: True
...
>>> if None: True
...
That's may not be all the possibilities. Lots of Python objects
can evaluate to false in a boolean context.
--
Neil Cerutti
The Pastor would appreciate it if the ladies of the congregation would lend
him their electric girdles for the pancake breakfast next Sunday morning.
--Church Bulletin Blooper
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Re: Checking if elements are empty
Lawrence D'Oliveiro wrote: if y[0]: >>> Not a good idea. >> Why not? > > Because there is a situation where your version of the test will fail even > if the first element of y is non-null. Such as? Seriously people, a little more verbosity wouldn't hurt here. This isn't a mystery game. /W -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
In message <[EMAIL PROTECTED]>, Wildemar Wildenburger wrote: > Lawrence D'Oliveiro wrote: > >> In message <[EMAIL PROTECTED]>, Chris >> Mellon wrote: >> >>> On 9/5/07, Steve Holden <[EMAIL PROTECTED]> wrote: Doran, Harold wrote: > > Is there a way to check if the first element of y is null? > len(y[0]) == 0 >>> Better spelled as >>> >>> if y[0]: >> >> Not a good idea. > > Why not? Because there is a situation where your version of the test will fail even if the first element of y is non-null. -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
Steven D'Aprano wrote: > On Fri, 07 Sep 2007 11:12:05 +0200, Wildemar Wildenburger wrote: > >> Lawrence D'Oliveiro wrote: >>> In message <[EMAIL PROTECTED]>, Chris >>> Mellon wrote: >>> On 9/5/07, Steve Holden <[EMAIL PROTECTED]> wrote: > Doran, Harold wrote: >> Is there a way to check if the first element of y is null? >> > len(y[0]) == 0 > Better spelled as if y[0]: >>> Not a good idea. >> Why not? > > > What happens if y is an empty list? > > An exception pops up, of course ;). It all depends on the (intended) semantics of the program; the original question sounds like that list is supposed to have at least one element at that point, so there is no problem. And if it's not then that has to be dealt with, but that is not the problem here. So the usual way to check if the first element of a list y is True in a boolean context is indeed "if y[0]:" (or for False of course "if not y[0]:"). /W -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
On Fri, 07 Sep 2007 11:12:05 +0200, Wildemar Wildenburger wrote: > Lawrence D'Oliveiro wrote: >> In message <[EMAIL PROTECTED]>, Chris >> Mellon wrote: >> >>> On 9/5/07, Steve Holden <[EMAIL PROTECTED]> wrote: Doran, Harold wrote: > Is there a way to check if the first element of y is null? > len(y[0]) == 0 >>> Better spelled as >>> >>> if y[0]: >> >> Not a good idea. > > Why not? What happens if y is an empty list? -- Steven. -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
Lawrence D'Oliveiro wrote: > In message <[EMAIL PROTECTED]>, Chris Mellon > wrote: > >> On 9/5/07, Steve Holden <[EMAIL PROTECTED]> wrote: >>> Doran, Harold wrote: Is there a way to check if the first element of y is null? >>> len(y[0]) == 0 >>> >> Better spelled as >> >> if y[0]: > > Not a good idea. Why not? /W -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
In message <[EMAIL PROTECTED]>, Chris Mellon wrote: > On 9/5/07, Steve Holden <[EMAIL PROTECTED]> wrote: >> Doran, Harold wrote: > >> > >> > Is there a way to check if the first element of y is null? >> > >> >> len(y[0]) == 0 >> >> would be the obvious way, assuming "null" means "the null string". >> > > Better spelled as > > if y[0]: Not a good idea. -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
> Doran, Harold wrote:
>
> I presume you meant
>
> x = ' \t\'ff'
>
>
> > Is there a way to check if the first element of y is null?
> >
You can use startswith() method of string objects.
if x.startswith(' '):
print True
--
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http://uthcode.sarovar.org
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Re: Checking if elements are empty
On 9/5/07, Chris Mellon <[EMAIL PROTECTED]> wrote: > > On 9/5/07, Steve Holden <[EMAIL PROTECTED]> wrote: > > Doran, Harold wrote: > > > > > > > Is there a way to check if the first element of y is null? > > > > > > > len(y[0]) == 0 > > > > would be the obvious way, assuming "null" means "the null string". > > > > Better spelled as > > if y[0]: Even better spelled as if not y[0]: ;-) -- > http://mail.python.org/mailman/listinfo/python-list > -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
On 9/5/07, Steve Holden <[EMAIL PROTECTED]> wrote: > Doran, Harold wrote: > > > > Is there a way to check if the first element of y is null? > > > > len(y[0]) == 0 > > would be the obvious way, assuming "null" means "the null string". > Better spelled as if y[0]: -- http://mail.python.org/mailman/listinfo/python-list
Re: Checking if elements are empty
Doran, Harold wrote:
> Dear list:
>
> Suppose I have a string as follows
>
> x = ' \t'ff'
> >>> x = ' \t'ff'
File "", line 1
x = ' \t'ff'
^
SyntaxError: invalid syntax
>>>
I presume you meant
x = ' \t\'ff'
> I can split this up as
>
> y = x.split('\t')
>
> Which gives
>
> [ ' ', 'ff']
>
> len(y)
> 2
>
> Is there a way to check if the first element of y is null?
>
len(y[0]) == 0
would be the obvious way, assuming "null" means "the null string".
If whitespace is significant that'll do, if it isn't then you may need
to use the strip() method to remove it before your scheck.
regards
Steve
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Skype: holdenweb http://del.icio.us/steve.holden
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