Type aliases in class definitions

[evhub]

Under MyPy 0.780, the code

from typing import List

class ABC:
    ABCInputType = List

x: ABC.ABCInputType = []

throws

test.py:6: error: Variable "test.ABC.ABCInputType" is not valid as a type
test.py:6: note: See https://mypy.readthedocs.io/en/latest/common_issues.html#variables-vs-type-aliases
Found 1 error in 1 file (checked 1 source file)

while

from typing import List

ABCInputType = List

x: ABCInputType = []

works just fine.

The use of type aliases in classes seems both like a pretty legitimate use case to me (consider for example the ability to have an input type for the class as an attribute of the class as above) and also seems consistent with both the typing documentation and mypy's documentation, neither of which seem to mention that type aliases can only appear at the top level (only that they need to be plain assignments, as they are in the above code).

[ilevkivskyi]

Yeah that doc section you linked is incomplete. The PR that added it closed the relevant issue but didn't address all points, in particular the special case about plain names at class scope creating variables.

cc @TH3CHARLie

Btw the fix is simply to use:

class ABC:
    ABCInputType = List[Any]

[evhub]

Btw the fix is simply to use:

What causes List[Any] to work there but List to fail? Shouldn't those be identical? And is there any equivalent of this fix that works for the more generic case where I just want some alias?

[ilevkivskyi]

And is there any equivalent of this fix that works for the more generic case where I just want some alias?

A trick that should work if you want to force type alias for plain name at class scope is use a single element union.

[TH3CHARLie]

Yeah that doc section you linked is incomplete. The PR that added it closed the relevant issue but didn't address all points, in particular the special case about plain names at class scope creating variables.

cc @TH3CHARLie

Btw the fix is simply to use:

class ABC:
    ABCInputType = List[Any]

I did recall that when doing the docs PR we did mention about the alias should appear at top level but somehow I must have forgotten to include that in the final version. I'll file a PR to fix that.

[tmke8]

The problem as stated in the issue above is solved now with PEP 613:

from typing import List, TypeAlias

class ABC:
    ABCInputType: TypeAlias = List

x: ABC.ABCInputType = []

-> no errors

However, this does not work in dataclasses (or any dataclass-like class), where you have to use ClassVar to exclude the variable from being used

from dataclasses import dataclass
from typing import ClassVar, List, TypeAlias

@dataclass
class ABC:
    ABCInputType: ClassVar[TypeAlias] = List

x: ABC.ABCInputType = []
reveal_type(x)  # Revealed type is "Any"

There's no error in mypy 0.942, but it thinks the type of x is Any.

[AlexWaygood]

The problem as stated in the issue above is solved now with PEP 613

Thanks! I'm closing this now since the original issue is now solved for most cases. Please feel free to open a new issue for the dataclasses- specific issue.