Cannot extend all protocols

[SimpleArt]

Related: #561

Previously subclassing typing.Iterable with typing.Protocol raised an error because typing.Iterable was not an actual protocol. This specific problem was fixed but there are other protocols which are not actual protocols, such as typing.AbstractSet.

My specific use-case is creating particularly spurred by the following line:

https://github.com/python/cpython/blob/144aaa74bbd77aee822ee92344744dbb05aa2f30/Lib/_collections_abc.py#L9

For example, if I wanted to extend collections.abc.Set to include the missing methods mentioned, I would approach it something like this:

# protocols.py
import typing


# Created for consistency and `isinstance` usage.
# Causes the mentioned error.
class AbstractSet(typing.AbstractSet, typing.Protocol):
    ...

# collections_abc.py
import protocols
import collections.abc

# Created to provide default implementations.
class AbstractSet(collections.abc.Set, protocols.AbstractSet):
    ...

It seems to me that the current fix for users is to simply not subclass the protocols and write out the entire protocol themselves.

[SimpleArt]

I just realized that:

• typing.AbstractSet and similar are deprecated.
• They were intended to be generic versions of their counterparts, not protocols.

So if you want protocols, you'll need to fully implement them from scratch.

[JelleZijlstra]

Yes, in general only the very simple (1-2 methods) abstract types were made into protocols. The more complex ones (Sequence, Mapping, etc.) aren't.