gh-108394: Fix subclassing annotated generic alias

[sobolevn]

I've decided to go in a different direction from the one suggested by @eltoder

Now, we reuse all __mro__ logic from generic aliases.

• Issue: Subclassing Annotated generic class fails #108394

[eltoder]

Can we add an error case with a SpecialForm that is not a GenericAlias, like NoReturn or Self?

[eltoder]

I think this fails in cases like:

class C(typing.Annotated[typing.Callable[[], int], 123]):
    pass

C.__bases__ should include Generic. Another similar case that already fails is

class C(typing.Generic[T], typing.Annotated[int, 123]):
    pass

C.__bases__ lost Generic.

[AlexWaygood]

@eltoder, your fist snippet works fine for me with @sobolevn's PR branch checked out:

Running Release|x64 interpreter...
Python 3.13.0a0 (heads/main:0e8b3fc718, Aug 28 2023, 13:48:35) [MSC v.1932 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import *
>>> class C(Callable[[], int]): ...
...
>>> C.__mro__
(<class '__main__.C'>, <class 'collections.abc.Callable'>, <class 'typing.Generic'>, <class 'object'>)
>>> class C(Annotated[Callable[[], int], 123]): ...
...
>>> C.__bases__
(<class 'collections.abc.Callable'>, <class 'typing.Generic'>)
>>> C.__mro__
(<class '__main__.C'>, <class 'collections.abc.Callable'>, <class 'typing.Generic'>, <class 'object'>)

Your second case does indeed look buggy, but as you say, the bug already exists on main. This is the behaviour we get on both this PR branch and the main branch:

Python 3.13.0a0 (heads/main:0e8b3fc718, Aug 28 2023, 13:48:35) [MSC v.1932 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import *
>>> T = TypeVar("T")
>>> class C(Generic[T], Annotated[int, 123]): ...
...
>>> C.__bases__
(<class 'int'>,)

[sobolevn]

Generic and Annotated together is not tested, please open a new issue about it.

[AlexWaygood]

Yes, I think we would need to special-case _AnnotatedAlias in _GenericAlias.__mro_entries__. The cause of the bug is this behaviour:

>>> from typing import *
>>> T = TypeVar("T")
>>> Generic[T].__mro_entries__((Generic[T], Annotated[int, 123]))
()

It's a separate change, unrelated to this one.

[eltoder]

@AlexWaygood sorry, off-by-one in the first example. Should be

class B: pass
class C(B, typing.Annotated[typing.Callable[[], int], 123]):
    pass

The point is that catching exceptions and setting i = 0 introduces subtle bugs.

[AlexWaygood]

The point is that catching exceptions and setting i = 0 introduces subtle bugs.

Indeed, good catch. Minimal repro:

>>> import types
>>> from typing import *
>>> class A: pass
...
>>> types.new_class('B', (A, Callable[[], int])).__mro__
(<class 'abc.B'>, <class '__main__.A'>, <class 'collections.abc.Callable'>, <class 'typing.Generic'>, <class 'object'>)
>>> types.new_class('B', (A, Annotated[Callable[[], int], 123])).__mro__
(<class 'abc.B'>, <class '__main__.A'>, <class 'collections.abc.Callable'>, <class 'object'>)

[AlexWaygood]

I think we should delegate to any __mro_entries__ method, if it exists, not just for _GenericAlias and GenericAlias:

Suggested change

	if isinstance(self.__origin__, (_GenericAlias, GenericAlias)):
	return self.__origin__.__mro_entries__(bases)
	if hasattr(self.__origin__, "__mro_entries__"):
	return self.__origin__.__mro_entries__(bases)

An example of an improvement this change would lead to: currently you get a nice error message if you try subclassing a NewType instance:

>>> from typing import *
>>> UserID = NewType("UserID", int)
>>> class Foo(UserID): pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 3084, in __init_subclass__
    raise TypeError(
TypeError: Cannot subclass an instance of NewType. Perhaps you were looking for: `Foo = NewType('Foo', UserID)`

But the nice error message goes away if you subclass Annotated[newtype_instance]:

>>> class Foo(Annotated[UserID, 'must be above 100']): pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: NewType.__init__() takes 3 positional arguments but 4 were give

If we make the change I'm suggesting here, we'll just delegate to NewType.__mro__, so we'll still get the nice error message:

>>> class Foo(Annotated[UserID, 'must be above 100']): ...
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 3084, in __init_subclass__
    raise TypeError(
TypeError: Cannot subclass an instance of NewType. Perhaps you were looking for: `Foo = NewType('Foo', UserID)`