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[Python-ideas] warn/error when using a method as boolean in ifs/whiles

Ethan Furman ethan at stoneleaf.us
Tue Oct 11 11:34:22 EDT 2016 
On 10/11/2016 07:00 AM, Steven D'Aprano wrote:
> On Tue, Oct 11, 2016 at 02:41:34PM +0200, Sven R. Kunze wrote:

>> on django-developers, an intriguing idea appeared:
>> https://groups.google.com/d/msg/django-developers/4bntzg1HwwY/HHHjbDnLBQAJ
>>
>> """
>> It seems to me that the default `method.__bool__` is undesirable in
>> Jinja2 templates. I do not know Jinja2 well enough, but maybe they could
>> benefit from a patch where `if`-statements give a warning/error when the
>> expression is a callable (with the default `FunctionType.__bool__`?
>> This would solve the issue not just for the methods you mention, but
>> more in general.
>
> That should be easy enough to do as a custom descriptor.
>
> But I would not like to see the default function or method __bool__
> raise a warning.

[...]

> So I think this is something that Django/Jinja2 should implement for its
> own methods that need it, it should not be a general feature of all
> Python functions/methods.

Agreed. Python is a /general/-purpose programming language.  We should not make changes to help one subset of users when those changes will harm another subset (and being flooded with false positives is harmful) -- particularly when easy customization is already available.

--
~Ethan~
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