Hi everyone,

Type variables allow to specify their variance at construction time, i.e variance is currently a property of a type variable. This is problematic for cases when a type variable is used outside of a generic class. For example,

T_co = TypeVar("T", covariant=True) def callback(value: T_co): ...

or

ListT = list[T_co]

I am curious, if we could instead deprecate the variance parameters and make variance local to the generic class definition. Specifically,

* all type variables are created invariant; * a type variable can be annotated as covariant and contravariant using unary + and - operators respectively; * these operators are only allowed when a type variable is referenced in the bases of a class definition.

For example, here

T = TypeVar("T") class Source(Generic[+T]): ...

the class Source is covariant in T.

Wdyt?

Sergei