Sorry, the formatting was terrible. I copy and pasted it from the original issue without really thinking about it.

Here's the same thing, but actually readable:

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. I've proposed this in issue 44941. 

Another possible implementation may be similar to a dataclass that implements __subclasshook__ for you with the desired method checks.

--

Finn