Python 3 类型注解和子类

Question

How do I refer to 'any object that subclasses a parent class' in Python type annotations?

Example: FooBase is an abstract base class, from which Foo1 , Foo2 , etc. are subclassed. I want the function to accept any descendant of FooBase . Will this do:

def do_something(self, bar:FooBase):
    pass

Or will this only accept an object of class FooBase , which of course is impossible given that FooBase is abstract? In that case, do I need to build a Union of all cases (please God I hope not!), or can I through some other way express this relationship abstractly?

Answer 1

Inheritance also applies to annotated types. Any instance of Foo which is a subtype of FooBase is also a valid object of the type FooBase . So you can pass a FooBase object but also a Foo object to the function.

If you want to limit the function to only subclasses of FooBar , you could take a look at the Type[C] construct: The type of class objects .

Answer 2

will this only accept an object of class FooBase ?

No, this will accept any subclasses too. This is also stated in the Theory of Type Hinting PEP, specifically the summary of Gradual Typing section :

A type t1 is consistent with a type t2 if t1 is a subtype of t2 . (But not the other way around.)

take a look at it for further pointers when dealing with type hints.

do I need to build a Union of all cases.

Even if you did, all subclasses will be eliminated from the Union and the subclasses are going to get skipped. Try creating the Union you mention:

typing.Union[Foo1, Foo2, FooBar]

and the result should be FooBar . The fact that it is an abstract class pays no difference here, Python itself uses many abstract classes in the typing module.

Take for example the Sized abc; hinting a function with Sized allows any virtual subclass (classes defining __len__ ) to be substituted:

def foo(obj: Sized): pass

foo([1, 2, 3, 4]) # ok
foo([2, 3, 4, 5]) # ok

Answer 3

Solved using TypeVar and PyCharm checker accepts it:

from pydantic import BaseModel
class MyModel(BaseModel):
    pass


from typing import TypeVar
BaseModelType = TypeVar('BaseModelType', bound='BaseModel')

async def do_smth(value: BaseModelType):
    pass

# ...

await do_smth(MyModel())