如何在Python中为子类指定更具体的泛型类型

Question

我正在尝试使已实现的泛型类型的子类在子类中具有更严格的泛型类型。简化后，我的代码如下：

>>> from typing import Generic, TypeVar
>>>
>>> T = TypeVar('T')
>>>
>>>
>>> class A:
...     pass
...
>>> class B(A):
...     pass
...
>>> class X(Generic[T]):
...     pass
...
>>> class Y(X[A]):
...     pass
...
>>> class Z(Y[B]):
...     pass
...
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    class Z(Y[B]):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/typing.py", line 251, in inner
    return func(*args, **kwds)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/typing.py", line 838, in __class_getitem__
    _check_generic(cls, params)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/typing.py", line 201, in _check_generic
    raise TypeError(f"{cls} is not a generic class")
TypeError: <class '__main__.Y'> is not a generic class

以下内容似乎有效，但我不确定这是否正确：

>>> class Z(Y, X[B]):
...     pass
...