Cannot declare the type of a type variable

[jaswilli]

After updating a project to version 0.720 I'm encountering a new error triggered by applying Final (from typing_extensions) to a TypeVar. A reproduction is below:

from typing import TypeVar
from typing_extensions import Final

# ok
MyType1 = TypeVar("MyType1")

# not ok
MyType2: Final = TypeVar("MyType2")

output:

repro.py:8: error: Cannot declare the type of a type variable
repro.py:8: error: "object" not callable

• mypy version: 0.720
• python version: 3.7.4
• Do you see the same issue after installing mypy from Git master?

Yes (mypy-0.730+dev.b405e05e220935dca6aa895045924ee68b435091)

[JelleZijlstra]

What would be the point of adding Final to a TypeVar assignment? Mypy already disallows assigning to MyType2.

[jaswilli]

What would be the point of adding Final to a TypeVar assignment? Mypy already disallows assigning to MyType2.

I didn't realize it was already disallowed by mypy, that's good to know. I simply annotated the TypeVars as Final because it seemed like a reasonable thing to do and then when I upgraded to 0.720 I got the error.

If the new behavior is actually correct and desired then feel free to close this issue.

[JelleZijlstra]

OK, I think the old behavior of accepting final was probably just an accident, but leaving to @msullivan or @ilevkivskyi (authors of the Final PEP) to close this.

[ilevkivskyi]

Yes, type variables are always implicitly final. The error message is poor, but we already have #1573 to track bad error messages for invalid type variables.