pythonsubclasstype-hintingpython-typing

Subclass in type hinting


I want to allow type hinting using Python 3 to accept sub classes of a certain class. E.g.:

class A:
    pass

class B(A):
    pass

class C(A):
    pass

def process_any_subclass_type_of_A(cls: A):
    if cls == B:
        # do something
    elif cls == C:
        # do something else

Now when typing the following code:

process_any_subclass_type_of_A(B)

I get an PyCharm IDE hint 'Expected type A, got Type[B] instead.'

How can I change type hinting here to accept any subtypes of A?

According to PEP 484 ("Expressions whose type is a subtype of a specific argument type are also accepted for that argument."), I understand that my solution (cls: A) should work?


Solution

  • When you specify cls: A, you're saying that cls expects an instance of type A. The type hint to specify cls as a class object for the type A (or its subtypes) uses typing.Type.

    from typing import Type
    def process_any_subclass_type_of_A(cls: Type[A]):
        pass
    

    From The type of class objects :

    Sometimes you want to talk about class objects that inherit from a given class. This can be spelled as Type[C] where C is a class. In other words, when C is the name of a class, using C to annotate an argument declares that the argument is an instance of C (or of a subclass of C), but using Type[C] as an argument annotation declares that the argument is a class object deriving from C (or C itself).