[Python-Dev] About [].append == [].append

[Ivan Pozdeev]

On 21.06.2018 23:40, Guido van Rossum wrote:
> I'm with Serhiy here, for mutable values I don't think the methods 
> should compare equal, even when the values do. For immutables I don't 
> care either way, it's an implementation detail.
>
In this light, methods rather shouldn't have a rich comparison logic at 
all -- at the very least, until we have a realistic use case and can 
flesh out the requirements for it.

In my previous message, I meant that if they do have that logic, the 
right way is what `method_richcompare' does. And that was apparently 
what the method's author (that you might be familiar with) was thinking 
in 
https://github.com/python/cpython/commit/47b9ff6ba11fab4c90556357c437cb4feec1e853 
-- and even then and there, they were hesitant about the feature's 
usefulness.

But Serhiy has just disproven that that is the right way which looks 
like the final nail into its coffin.

> On Thu, Jun 21, 2018, 12:55 Serhiy Storchaka <storchaka at gmail.com 
> <mailto:storchaka at gmail.com>> wrote:
>
>     21.06.18 14:25, Jeroen Demeyer пише:
>     > Currently, we have:
>     >
>     >  >>> [].append == [].append
>     > False
>     >
>     > However, with a Python class:
>     >
>     >  >>> class List(list):
>     > ....     def append(self, x): super().append(x)
>     >  >>> List().append == List().append
>     > True
>
>     I think this is a bug. These bound methods can't be equal because
>     they
>     have different side effect.
>
>     The use case for using "is" for __self__ is described by the OP of
>     issue1617161. I don't know use cases for using "==".
>
>     There is a related problem of hashing. Currently
>     bound methods are not hashable if __self__ is not hashable. This
>     makes
>     impossible using them as dict keys.
>
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-- 
Regards,
Ivan

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