T = TypeVar('T')
T_co = TypeVar('T_co', covariant=True)
T_contra = TypeVar('T_contra', contravariant=True)

class Box(Protocol[T_co]):
    def content(self) -> T_co:
        ...

box: Box[float]
second_box: Box[int]
box = second_box  # This is OK due to the covariance of 'Box'.

class Sender(Protocol[T_contra]):
    def send(self, data: T_contra) -> int:
        ...

sender: Sender[float]
new_sender: Sender[int]
new_sender = sender  # OK, 'Sender' is contravariant.

class Proto(Protocol[T]):
    attr: T  # this class is invariant, since it has a mutable attribute

var: Proto[float]
another_var: Proto[int]
var = another_var  # Error! 'Proto[float]' is incompatible with 'Proto[int]'.