I do not understand error like

Type variable “T” is unbound

Here a concrete example:

from typing import Dict, Type, Callable, TypeVar


class Sup:
    ...


class A(Sup):
    ...


class B(Sup):
    ...


def get_A(a: A) -> int:
    return 1


def get_B(b: B) -> int:
    return 2


T = TypeVar("T", bound=Sup)

f: Dict[Type[T], Callable[[T], int]] = {A: get_A, B: get_B}

mypy returns:

error: Type variable "foo.T" is unbound  [valid-type]
note: (Hint: Use "Generic[T]" or "Protocol[T]" base class to bind "T" inside a class)
note: (Hint: Use "T" in function signature to bind "T" inside a function)
error: Dict entry 0 has incompatible type "type[A]": "Callable[[A], int]"; expected "type[T?]": "Callable[[T], int]"  [dict-item]
error: Dict entry 1 has incompatible type "type[B]": "Callable[[B], int]"; expected "type[T?]": "Callable[[T], int]"  [dict-item]

My understanding (which may be incorrect), is that mypy complains because it can not guarantee that later on, no tuple (key, value) not following the constrain on T will be added. For example:

# not a subclass of Sup !
class D:
  ...

def get_D(d: D)->int:
  return 2

f[D] = get_D  # D is not a subclass of Sup, so violation of the type of f

Question:

• is my understanding of the error correct ? If not, could an explanation be provided ?

• if my understanding is correct, why would the error occur here:

f: Dict[Type[T], Callable[[T], int]] = {A: get_A, B: get_B}

and not here ?

# This is the line not respecting the type of f !
f[D] = get_D