[issue44941] Add check_methods function to standard library

[Finn Mason]

Finn Mason  added the comment:

Thank you for the suggestions.

>From what I could tell, the cited python-ideas discussion was more about 
>checking the signature of a method. However, an issue cited by the ex-BDFL 
>(issue9731) was helpful. It's a similar idea that verifies that a class 
>implements an ABC. However, I'm looking at a function that simplifies writing 
>the ABC itself.

I will post the idea to python-ideas and see what people think.

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[issue44941] Add check_methods function to standard library

[Éric Araujo]

Éric Araujo  added the comment:

FYI there was a ticket and a discussion before: 
https://mail.python.org/archives/list/python-id...@python.org/thread/FCGDSVWHIJHBYKS2O4RHZVLXCGSGBLQH/#FCGDSVWHIJHBYKS2O4RHZVLXCGSGBLQH

Should be reviewed to see the arguments made and determine if things have 
changed to reopen the question!

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nosy: +eric.araujo

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[issue44941] Add check_methods function to standard library

[Ken Jin]

Ken Jin  added the comment:

@finnjavier08 sorry, I'd misinterpreted your original message. Thanks for 
clarifying your intent!

> I'd be happy to do the implementation and pull request once approval is given 
> and a module is decided on.

I can't comment because I'm not an expert on collections.abc/abc and friends, 
but you might want to post your idea on the python-ideas mailing list or 
https://discuss.python.org/c/ideas/6 to get other opinions first. I have the 
feeling more people read the former.

--
nosy: +kj

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[issue44941] Add check_methods function to standard library

[Finn Mason]

Finn Mason  added the comment:

I strongly feel that `check_methods` shouldn't be in collections.abc, even 
though that's where it's originally found, because it's not related 
specifically to collections but to ABCs and classes in general. I would prefer 
for it to be implemented in `abc` or similar.

I'm reverting to the original title until a module is decided on.

I'd be happy to do the implementation and pull request once approval is given 
and a module is decided on.

--
title: Add check_methods function to collections.abc in standard library -> Add 
check_methods function to standard library

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[issue44941] Add check_methods function to standard library

[Finn Mason]

New submission from Finn Mason :

In _collections_abc.py is a private function titled `_check_methods`. It takes 
a class and a number of method names (as strings), checks if the class has all 
of the methods, and returns NotImplemented if any are missing. The code is 
below:

```
def _check_methods(C, *methods):
mro = C.__mro__
for method in methods:
for B in mro:
if method in B.__dict__:
if B.__dict__[method] is None:
return NotImplemented
break
else:
return NotImplemented
return True
```

This is an incredibly convenient function (referred to as check_methods here on 
out) for creating abstract base classes, and is much simpler than using 
`hasattr` for each method you want to check. For example:

```
>>> from abc import ABCMeta
>>> # Without check_methods
>>> class A(metaclass=ABCMeta):
... @classmethod
... def __subclasshook__(cls, subclass):
... return (hasattr(subclass, 'foo') and
... callable(subclass.foo) and
... hasattr(subclass, 'bar') and
... callable(subclass.bar) or
... NotImplemented)
...
>>> # With check_methods
>>> class B(metaclass=ABCMeta):
... @classmethod
... def __subclasshook(cls, subclass):
... return check_methods(subclass, 'foo', 'bar')
...
>>>
```

This would be a great function to add to the standard lib, perhaps in the `abc` 
module.

One problem with `check_methods` as defined in _collections_abc.py is that it 
doesn't check if the name is callable. Also, type hints and more readable 
variables may be desirable. The final code, if implemented, may look something 
like this:

```
# In imports section: from typing import Literal
def check_methods(Class: type, *methods: str) -> Literal[True, NotImplemented]:
"""Check if class `Class` has methods `methods`."""
mro = Class.__mro__
for method in methods:
for Base in mro:
if (attr := getattr(Base, method, None)) is not None:
if not callable(attr):
return NotImplemented
break
else:
return NotImplemented
return True
```

Again, this would be a great function to add to the `abc` module or a similar 
one.

--
components: Library (Lib)
messages: 399814
nosy: finnjavier08
priority: normal
severity: normal
status: open
title: Add check_methods function to standard library
type: enhancement

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