An example of how to annotate cls return type class in inheritance using mypy in order to escape error `AttributeError: type object 'A' has no attribute 'bazz'`

inheritance_and_cls_annotation.py

"""
An example of how to annotate cls return type class in inheritance using mypy
in order to escape error `AttributeError: type object 'A' has no attribute 'bazz'`

Here is what we done:
* The type variable TA is used to denote that return values might be an
instances of subclasses of A.
* Specify that A is an upper bound for TA.
Specifying bound means that TA will only be A or one of its subclasses.
This is needed to properly restrict the types that are allowed.
* The typing.Type[] construct is the typing equivalent of type().
We need it to note that the class method expects a class
and returns an instance of that class.
"""

from __future__ import annotations
from typing import TypeVar, Type

TA = TypeVar("TA", bound="A")


class A:
    def __init__(self, param: str) -> None:
        self.param = param

    def bar(self) -> str:
        return "bar"

    @classmethod
    def bazz(cls: Type[TA]) -> TA:
        return cls("bazz")


class B(A):
    def foo(self) -> str:
        return "foo"


a = A.bazz()
print(a.param)
print(a.bar())

b = B.bazz()
print(b.param)
print(b.foo())
print(b.bar())