Support `type.__orig_bases__` as defined in PEP 560

[huntzhan]

__orig_bases__ is defined in PEP 560. Currently class type does not include this symbol. This symbol is helpful for writing type reflection code. It would be great if this symbol is supported.

related pyright issue: microsoft/pyright#3442

[AlexWaygood]

We can add this, for sure. A few thoughts, though.

(1) If you're using this in your code, I'd be wary of this sentence in PEP 560:

NOTE: These two method names are reserved for use by the typing module and the generic types machinery, and any other use is discouraged. The reference implementation (with tests) can be found in [4], and the proposal was originally posted and discussed on the typing tracker, see [5].

(2) The attribute does not exist on all type objects, only on subclasses of typing.Generic (so I think it should be added as a ClassVar on typing.Generic). On Python 3.9, for example:

>>> class Foo: ...
... 
>>> Foo.__orig_bases__
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AttributeError: type object 'Foo' has no attribute '__orig_bases__'
>>> from typing import TypeVar, Generic
>>> T = TypeVar('T')
>>> class Bar(Generic[T]): ...
... 
>>> Bar.__orig_bases__
(typing.Generic[~T],)
>>> list[int].__orig_bases__
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AttributeError: type object 'list' has no attribute '__orig_bases__'

(3) I'm not 100% sure what the type should be. I know Eric said in the pyright issue that he thought Sequence[type] would be a good choice. But check it out:

>>> from typing import TypeVar, Generic
>>> T = TypeVar('T')
>>> class Bar(Generic[T]): ...
... 
>>> Bar.__orig_bases__
(typing.Generic[~T],)
>>> isinstance(_[0], type)
False

[AlexWaygood]

Ah, but then on the other hand, we don't have typing.Generic as a class currently in typing.pyi, so adding a ClassVar to it in the stub might be slightly tricky:

typeshed/stdlib/typing.pyi

Line 516 in 5c13f8b

Generic: _SpecialForm = ...

[hmc-cs-mdrissi]

I'd probably call them typeforms instead of types. If we had typeform pep I think I'd do tuple[Typeform, ...]. It's similar to how list is a type, but list[int] is not a type (or how Union[int, str] fails to be a type). Most of the code I use that does orig_bases introspection is for stuff like,

class Container(Generic[T]):
  @classmethod
  def object_type(cls) -> T:
    return cls.__orig_bases__[0]

Here the type of object_type can be any valid typeform. Union[int, str], List[str], etc are all valid values.

Without typeform pep I think type serves as best approximation.

[huntzhan]

I'd probably call them typeforms instead of types. If we had typeform pep I think I'd do tuple[Typeform, ...]. It's similar to how list is a type, but list[int] is not a type (or how Union[int, str] fails to be a type). Most of the code I use that does orig_bases introspection is for stuff like,

class Container(Generic[T]):
  @classmethod
  def object_type(cls) -> T:
    return cls.__orig_bases__[0]

Here the type of object_type can be any valid typeform. Union[int, str], List[str], etc are all valid values.

Without typeform pep I think type serves as best approximation.

Probably this example ?

from typing import get_args

class Container(Generic[T]):
  @classmethod
  def object_type(cls) -> T:
    return get_args(cls.__orig_bases__[0])[0]