Python typing signature for instance of subclass?

Consider:

from __future__ import annotations    class A:      @classmethod      def get(cls) -> A:          return cls()    class B(A):      pass    def func() -> B: # Line 12      return B.get()  

Running mypy on this we get:

$ mypy test.py  test.py:12: error: Incompatible return value type (got "A", expected "B")  Found 1 error in 1 file (checked 1 source file)  

Additionally, I have checked to see if old-style recursive annotations work. That is:

# from __future__ import annotations    class A:      @classmethod      def get(cls) -> "A":  # ...  

...to no avail.

Of course one could do:

from typing import cast    def func() -> B: # Line 12      return cast(B, B.get())  

Every time this case pops up. But I would like to avoid doing that.

How should one go about typing this?

https://stackoverflow.com/questions/65963177/python-typing-signature-for-instance-of-subclass January 30, 2021 at 07:44AM