I'm trying to define a custom generic dict, whose keys are of type T_key and values are of type T_val.
I also want to put constraints on T_key and T_val, such that T_key can only be of type A or B or their subclass.
How do I accomplish this?
from typing import TypeVar, Generic
class A: ...
class B: ...
class Asub(A): ...
class Bsub(B): ...
T_key = TypeVar('T_key', A, B, covariant=True)
T_val = TypeVar('T_val', A, B, covariant=True)
class MyDict(Generic[T_key, T_val]): ...
w: MyDict[ A, B]
x: MyDict[ A, Bsub]
y: MyDict[Asub, B]
z: MyDict[Asub, Bsub]
When I try to check this, mypy gives errors on annotations of x, y and z. Only the annotation for w works as expected.
generic.py:17: error: Value of type variable "T_val" of "MyDict" cannot be "Bsub"
generic.py:18: error: Value of type variable "T_key" of "MyDict" cannot be "Asub"
generic.py:19: error: Value of type variable "T_key" of "MyDict" cannot be "Asub"
generic.py:19: error: Value of type variable "T_val" of "MyDict" cannot be "Bsub"
I don't understand why Asub is not a valid type for T_key even with covariant=True specified.
What am I missing here?
mypy version: 0.630
That's not what covariance means. With a covariant type variable T and a generic class Foo[T], instances of Foo[Subclass] are also considered instances of Foo[Superclass]. Covariance has no effect on what types may be substituted for T.
If your B were defined as
class B(A): ...
instead of
class B: ...
, then a value of type MyDict[B, B] would be considered a valid value of type MyDict[A, A] by static type checkers due to covariance. You would still not be able to create a value of type MyDict[ASub, BSub], because the only valid values of your type variables are A and B.
The concept you're looking for is bounded type variables, using the bound keyword argument, not constrained type variables. It looks like you can specify a union as the bound, which comes as quite a surprise to me, so declaring the type variables as
T_key = TypeVar('T_key', bound=Union[A, B])
T_val = TypeVar('T_val', bound=Union[A, B])
should work.