How can I define a generic covariant function in Python?

Question

I want to define a function example that takes an argument of type Widget or anything that extends Widget and returns the same type as the argument. So if Button extends Widget , calling example(Button()) returns type Button .

I tried the following:

T_co = TypeVar('T_co', Widget, covariant=True)

def example(widget: T_co) -> T_co:
  ...

However the type checker (Pyright) ignores the covariance. Upon further research I found a note in PEP 484 :

Note: Covariance or contravariance is not a property of a type variable, but a property of a generic class defined using this variable. Variance is only applicable to generic types; generic functions do not have this property. The latter should be defined using only type variables without covariant or contravariant keyword arguments.

However if I try to define a generic function without the covariant argument as specified in the note:

T_co = TypeVar('T_co', Widget)

def example(widget: T_co) -> T_co:
  ...

I can only pass values of type Widget to the function (not Button ).

How can I achieve this?

Answer 1

I was able to find the answer in the MyPy docs . Turns out I was looking for bound , not covariant . This can be done like so:

T = TypeVar('T', bound=Widget)

def example(widget: T) -> T:
  ...