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

[Steven D'Aprano]

On Tue, Oct 11, 2016 at 02:41:34PM +0200, Sven R. Kunze wrote:
> Hey python-ideas,
> 
> 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. Consider processing a sequence of functions/methods, 
skipping any which are None:

for func in callables:
    if func is not None:
        func(some_arg)

I often written code like that. Now imagine that somebody reasons that 
since all functions and methods are truthy, and None if falsey, we can 
write the code as:

for func in callables:
    if func:
        func(some_arg)

That's perfectly reasonable code too, and it should be purely a matter 
of taste whether you prefer that or the first version. But with this 
suggestion, we get flooded by spurious warnings.

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.


-- 
Steve