[Tutor] if n == 0 vs if not n
Luke Paireepinart
rabidpoobear at gmail.com
Tue Oct 6 03:13:40 CEST 2009
On Tue, Oct 6, 2009 at 2:40 AM, Vern Ceder <vceder at canterburyschool.org>wrote:
> Dave Angel wrote:
>
> Now in this case where it is only used as boolean checks which would be
>>>> the most pythonic way if writing these checks?
>>>>
>>>>
>>> The shorter version may be preferable, but it doesn't generally give the
>> same results. Without knowing the possible data, these substitutions are
>> not safe.
>>
>> For example, replacing "if not n == 0" with "if n"
>>
>> will give different results for values of "", [] and so on. It WILL
>> work if you know that n is an int or float, however.
>>
>> DaveA
>>
>
> True, I took the OP's statement that they were to be used "only as boolean
> checks" to mean that there was no type mixing going on. Personally, I would
> say that checking a list or string for equality (or lack thereof) with 0 is
> even less "preferable". ;)
>
> Otherwise, one would at least prefer "if n != 0" to "if not n == 0", I
> would think.
>
Actually, I just realized that "not" has higher precedence than "==" so this
is really checking if (not n) is equal to 0, not if (n == 0) is (not) True.
So essentially "not n" is evaluated and is turned into a bool, which is
then compared to 0, which is the same as False in a boolean context, but "n
!= 0" is comparing n to 0, where n may not be the same type (eg. a string).
I'm not sure if there's a situation where this difference matters, but I
feel like there might be.
Anyone have some counter-examples to n != 0 being the same as "not n == 0"?
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