python - Subclass in type hinting

Question

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 this (https://www.python.org/dev/peps/pep-0484/#type-definition-syntax, "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?

Is Python also raising this error ? Otherwise, it's maybe a bug in PyCharm ! — jpic
For me I tried on python console and It is working fine. Might be possible bug in PyCharm.. — Nirmi
Yes it's working, the type hinting is optional as far as I know. Still I want the user of the method to explicitly know what classes are intended for usage there. — user1211030

Ashwini Chaudhary Ashwini Chaudhary · Accepted Answer · 2017-09-07T09:09:28

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).

python - Subclass in type hinting

1 Answers