How can I set the same type as class in method's parameter following PEP484?

Question

My question is related with the new Python's type hints. I'm trying to add a type hint in an object's method who has a parameter of the same type of the object, but PyCharm are marking me as error (unresolved reference 'Foo'). The problem is as follows:

class Foo:

    def foo_method(self, other_foo: Foo):
        return "Hello World!"

So the question is how to define the type of other_foo parameter properly. Maybe __class__ is correct?

Answer 1

Inside of the class, the class is not defined yet, causing a NameError (and PyCharm to complain).

To get around this, use a forwarding reference:

class Foo:
    def foo_method(self, other_foo: "Foo"):
        return "Hello World!"

Basically, if a type annotations is a string, it is evaled when the type is requested (in the global scope rather than the scope of the class Foo), so it can evaluate to the Foo class.

---

You can also use the annotations feature, which will turn all annotations into strings:

from __future__ import annotations

class Foo:
    def foo_method(self, other_foo: Foo):
        return "Hello World!"

# Foo.foo_method.__annotations__ == {'other_foo': 'Foo'}
# typing.get_type_hints(Foo.foo_method) == {'other_foo': Foo}

This is available since 3.7, and was slated to become the default behaviour in 3.10 but was postponed.