Re: create boolean
On 2009-03-09, Andre Engels wrote: > On Mon, Mar 9, 2009 at 12:48 AM, Grant Edwards wrote: > >> I didn't say that he hadn't authorized that assumption. ??I >> just said that the code does rely on such an assumption. ??In >> my experience, assumptions like that result broken code down >> the road. > > And assumptions like "when assumptions fail, it is save to go by the > letter of the requirement" don't? If the requirement is wrong, then it doesn't really matter what you assume. -- Grant Edwards grante Yow! OVER the underpass! at UNDER the overpass! visi.comAround the FUTURE and BEYOND REPAIR!! -- http://mail.python.org/mailman/listinfo/python-list
Re: create boolean
Lie Ryan wrote:
> Fencer wrote:
>> Hi, I need a boolean b to be true if the variable n is not None and
>> not an empty list, otherwise b should be false.
>> I ended up with:
>> b = n is not None and not not n
>> which seems to work but is that normally how you would do it?
>> It can be assumed that n is always None or a list that might be empty
>>
>> - Fencer
>
> The literal translation of that would be:
>
> if n is not None and n != []:
> b = True
> else:
> b = False
>
> it is a bit verbose, so one might want to find something shorter
>
> b = True if n is not None and n != [] else False
>
> I always feel if and in-line if to be easier and more readable than
> short-circuited operations.
So, in what significant way does this differ from
b = n is not None and n != []
? Just for kicks, let's take a look at that:
# py2.2+ ... not py3 ?1.5.2? ?1.4?
print "%7s %7s %7s %7s %7s %7s" % (
"a", "b", "bool(a)", "bool(b)", "formula1", "formula2")
formula1 = "True if n is not None and n != [] else False"
formula2 = "n is not None and n != []"
print 64 * "-"
for a in None, [], 0, 33, "", "banana", {}, {1: 'one'}:
for b in "-":
n = a
print "%7s %7s %7s %7s %7s %7s" % (
a, b, bool(a), bool(b),
eval(formula1), eval(formula2))
Those who run the program may see the results. I hope the small attached
graphic will be acceptable to them so those with no interpreter handy
may also see the results. If you do have an interpreter you are, of
course, free to vary the formulae, or add more for comparison.
regards
Steve
PS: Indeed b *is* completely redundant. I just adapted a program I had
lying around ...
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Re: create boolean
On Mon, Mar 9, 2009 at 12:48 AM, Grant Edwards wrote: > I didn't say that he hadn't authorized that assumption. I just > said that the code does rely on such an assumption. In my > experience, assumptions like that result broken code down the > road. And assumptions like "when assumptions fail, it is save to go by the letter of the requirement" don't? -- André Engels, andreeng...@gmail.com -- http://mail.python.org/mailman/listinfo/python-list
Re: create boolean
On 2009-03-08, Rhodri James wrote: b = (n is not None) and (n != []) >>> >>> The second comparison isn't actually necessary, since an >>> empty list is True and a non-empty one False. >>> >>>b = (n is not None) and n >>> >>> Putting the comparison in does make the code slightly less >>> "magic", though, so it's not a bad idea to do it! >> >> Putting in the second comparison in makes the code match the >> stated requirement. Otherwise you have to start making >> assumptions about what n might be besides None or the empty >> list. > > The OP stated that we *could* assume that n was None or a > list, so I stand by what I said. I didn't say that he hadn't authorized that assumption. I just said that the code does rely on such an assumption. In my experience, assumptions like that result broken code down the road. -- Grant -- http://mail.python.org/mailman/listinfo/python-list
Re: create boolean
On Sat, 07 Mar 2009 05:03:08 -, Grant Edwards wrote: On 2009-03-07, Rhodri James wrote: On Fri, 06 Mar 2009 15:34:08 -, Grant Edwards wrote: On 2009-03-06, Fencer wrote: Hi, I need a boolean b to be true if the variable n is not None and not an empty list, otherwise b should be false. I ended up with: b = n is not None and not not n I'd do it like this: b = (n is not None) and (n != []) The second comparison isn't actually necessary, since an empty list is True and a non-empty one False. b = (n is not None) and n Putting the comparison in does make the code slightly less "magic", though, so it's not a bad idea to do it! Putting in the second comparison in makes the code match the stated requirement. Otherwise you have to start making assumptions about what n might be besides None or the empty list. The OP stated that we *could* assume that n was None or a list, so I stand by what I said. -- Rhodri James *-* Wildebeeste Herder to the Masses -- http://mail.python.org/mailman/listinfo/python-list
Re: create boolean
Scott David Daniels wrote: Lie Ryan wrote: Fencer wrote: The literal translation of that would be: if n is not None and n != []: b = True else: b = False it is a bit verbose, so one might want to find something shorter b = True if n is not None and n != [] else False I always feel if and in-line if to be easier and more readable than short-circuited operations. How about: b = None is not n != [] It is amazing to think about how rarely we consider is / is not as a comparison operator. Also, and more reasonably, we often don't consider "chaining" comparisons that are intransitive. The fact that chaining operator is possible doesn't mean it must be used. The only place I would use chained operator is for comparing ranges of integer: 5 < x < 10, other than that, its use often reduce readability. -- http://mail.python.org/mailman/listinfo/python-list
Re: create boolean
On Sat, Mar 7, 2009 at 6:03 AM, Grant Edwards wrote: > Putting in the second comparison in makes the code match the > stated requirement. Otherwise you have to start making > assumptions about what n might be besides None or the empty > list. But the stated requirement already assumes that n is either None or a list. The outcome is simply undefined when used on something that is not None or a list. And it feels more in line with Python philosophy, in particular with duck typing, to have 'list-like objects' (like sets or tuples) behave like lists. -- André Engels, andreeng...@gmail.com -- http://mail.python.org/mailman/listinfo/python-list
Re: create boolean
Lie Ryan wrote: Fencer wrote: The literal translation of that would be: if n is not None and n != []: b = True else: b = False it is a bit verbose, so one might want to find something shorter b = True if n is not None and n != [] else False I always feel if and in-line if to be easier and more readable than short-circuited operations. How about: b = None is not n != [] It is amazing to think about how rarely we consider is / is not as a comparison operator. Also, and more reasonably, we often don't consider "chaining" comparisons that are intransitive. --Scott David Daniels scott.dani...@acm.org -- http://mail.python.org/mailman/listinfo/python-list
Re: create boolean
Fencer writes: > Hi, I need a boolean b to be true if the variable n is not None and > not an empty list, otherwise b should be false > It can be assumed that n is always None or a list that might be empty b = bool(n) -- http://mail.python.org/mailman/listinfo/python-list
Re: create boolean
Fencer wrote: Hi, I need a boolean b to be true if the variable n is not None and not an empty list, otherwise b should be false. I ended up with: b = n is not None and not not n which seems to work but is that normally how you would do it? It can be assumed that n is always None or a list that might be empty - Fencer The literal translation of that would be: if n is not None and n != []: b = True else: b = False it is a bit verbose, so one might want to find something shorter b = True if n is not None and n != [] else False I always feel if and in-line if to be easier and more readable than short-circuited operations. -- http://mail.python.org/mailman/listinfo/python-list
Re: create boolean
On 2009-03-07, Rhodri James wrote: > On Fri, 06 Mar 2009 15:34:08 -, Grant Edwards wrote: > >> On 2009-03-06, Fencer wrote: >> >>> Hi, I need a boolean b to be true if the variable n is not >>> None and not an empty list, otherwise b should be false. >> >>> I ended up with: >> >>> b = n is not None and not not n >> >> I'd do it like this: >> >> b = (n is not None) and (n != []) > > The second comparison isn't actually necessary, since an > empty list is True and a non-empty one False. > >b = (n is not None) and n > > Putting the comparison in does make the code slightly less > "magic", though, so it's not a bad idea to do it! Putting in the second comparison in makes the code match the stated requirement. Otherwise you have to start making assumptions about what n might be besides None or the empty list. -- Grant -- http://mail.python.org/mailman/listinfo/python-list
Re: create boolean
On Fri, 06 Mar 2009 15:34:08 -, Grant Edwards wrote: On 2009-03-06, Fencer wrote: Hi, I need a boolean b to be true if the variable n is not None and not an empty list, otherwise b should be false. I ended up with: b = n is not None and not not n I'd do it like this: b = (n is not None) and (n != []) The second comparison isn't actually necessary, since an empty list is True and a non-empty one False. b = (n is not None) and n Putting the comparison in does make the code slightly less "magic", though, so it's not a bad idea to do it! -- Rhodri James *-* Wildebeeste Herder to the Masses -- http://mail.python.org/mailman/listinfo/python-list
Re: create boolean
On 2009-03-06, Fencer wrote:
> Hi, I need a boolean b to be true if the variable n is not
> None and not an empty list, otherwise b should be false.
> I ended up with:
> b = n is not None and not not n
I'd do it like this:
b = (n is not None) and (n != [])
Your code doesn't meet your stated requirement. Your code
incorrectly evaluates to False if n is any of the following:
(,)
{}
False
0
0.0
Your stated requirement is for b to be True for all those cases.
> which seems to work
>
> but is that normally how you would do it?
>
> It can be assumed that n is always None or a list that might
> be empty
Well then, that's different. :)
Still, I'd do it my way. I don't like assumptions.
--
Grant Edwards grante Yow! If Robert Di Niro
at assassinates Walter Slezak,
visi.comwill Jodie Foster marry
Bonzo??
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Re: create boolean
Fencer schrieb: > Hi, I need a boolean b to be true if the variable n is not None and not > an empty list, otherwise b should be false. > I ended up with: > b = n is not None and not not n > which seems to work but is that normally how you would do it? > It can be assumed that n is always None or a list that might be empty b = bool(n) Christian -- http://mail.python.org/mailman/listinfo/python-list
create boolean
Hi, I need a boolean b to be true if the variable n is not None and not an empty list, otherwise b should be false. I ended up with: b = n is not None and not not n which seems to work but is that normally how you would do it? It can be assumed that n is always None or a list that might be empty - Fencer -- http://mail.python.org/mailman/listinfo/python-list
