Get class of a generic class's `TypeVar`

generic typevar

Source

• https://discuss.python.org/t/inside-a-generic-class-how-to-get-the-class-of-type-variable/20792
• https://stackoverflow.com/a/50101934/12780516
• https://github.com/python/cpython/blob/374abded070b861cc389d509937344073193c36a/Lib/types.py#L169

from typing import Generic, TypeVar
 
_T = TypeVar("_T")
 
class MyGenericClass(Generic[_T]):
    def __init__(self) -> None:
        TType: type[_T] = type(self).__orig_bases__[0].__args__[0]
        t = TType(...)
 
        # the naive approach of just using _T does not work, because it's just a typing annotation
        t = _T(...)  # raises `TypeError: 'TypeVar' object is not callable`

• as an alternative we can use get_original_bases() from the typing module (only available from Python 3.12; use typing_extensions for earlier versions instead) in combination with get_args() for a more idiomatic version

from typing import get_args, get_original_bases
 
# ...
TType: type[_T] = get_args(get_original_bases(type(self)))

• note that this might not work in all cases because get_original_bases() uses cls.__dict__.get("__orig_bases__", cls.__bases__) internally, which is not functionally equivalent (at least in Python 3.10)
• this can be mitigated by a custom implementation of get_original_bases()

from typing import Iterable
 
def get_original_bases(cls: type, /) -> Iterable[type]:
    """
    Return the class's "original" bases, similar to `typing.get_original_bases()` for Python 3.12+, but not using
    `__orig_bases__` from the class's `__dict__` since this seems to not always be available that way
    (`cls.__orig_bases__` is, though).
 
    Parameters
    ----------
    cls : type
        The class to get the original bases for
 
    Returns
    -------
    Iterable[type]
        The original bases of the class
    """
 
    return getattr(cls, "__orig_bases__", cls.__bases__)