[Python-ideas] Fwd: Boolean behavior of None

[Ilkka Pelkonen]

Thank you all. It was just that when I started with Python, everything
worked right like I expected, and I found the ways to do anything I've
needed all the way until today, so when I came across with this, it
appeared to me a clear bug in the language/interpreter. Casting to bool is
indeed a good solution and practice, and I do now agree that there's no
point in changing the language - like Antoine said, we're talking control
flow operators here, not exactly boolean. (This might be a good addition to
the documentation.)

Thank you Ned for the warm welcome and everyone for your input. I hope to
be able to contribute in the future. :)

Best Regards,
Ilkka


On Thu, Jan 17, 2013 at 6:07 PM, Mathias Panzenböck <
grosser.meister.morti at gmx.net> wrote:

> This change would break a lot of existing code and would make Python
> awkwardly stand out from all
> other modern dynamically typed languages (e.g. Ruby and JavaScript). You
> often write things like
> this:
>
>         def foo(bar=None):
>                 bar = bar or []
>                 ...
>
> Or:
>
>         obj = obj and obj.property
>
> The proposed change would needlessly complicate these things and break
> existing code. Forcing a
> bool type really doesn't require that much code (bool(expr)) and is good
> practise anyway.
>
>
> On 01/17/2013 01:44 PM, Ilkka Pelkonen wrote:
>
>> Hi all,
>> I ran into an issue in expression evaluation with Python for Windows
>> 2.7.3. Consider the following code:
>>
>> expected_result = (expected_string != 'TRUE') # Boolean
>> element = find_element() # Can return None or an instance of Element
>> flag = (element and element.is_visible())
>> if flag == expected_result:
>> ..# Ok
>> ..return
>> # Otherwise perform some failure related stuff.
>>
>> This code does not work. What happens on the 'flag' assignment row, is
>> that if 'element' is None, the expression returns None, not False. This
>> makes the if comparison to fail if expected_result is False, since boolean
>> False is not None.
>>
>> To me as a primarily C++ programmer it seems there could be two different
>> changes here, either change the behavior of the 'and' expression, forcing
>> it to return Boolean even if the latter part is not evaluated, and/or make
>> the comparison "False == None" return True. Although potentially complex,
>> I'd
>> myself go for the first approach. It seems to me more logical that False
>> != None than an 'and' expression returning non-boolean. Also the latter
>> change might require people change their code, while the former should not
>> require any modifications.
>>
>> This behavior probably results in lots of errors when people like me,
>> used to more traditional languages, take on Python in a serious manner. I
>> like the concept 'pythonic', and am trying to apply it to practice like
>> above.
>>
>> Hoping to hear your thoughts,
>> Regards,
>>
>> Ilkka Pelkonen
>>
>>
>>
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