Hello,
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
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