Type annotation error for generic list that includes union

[vishwa-raj]

Environment data

• Language Server version: 2021.6.3
• OS and version: win32 x64
• Python version (and distribution if applicable, e.g. Anaconda):
• python.analysis.indexing: undefined
• python.analysis.typeCheckingMode: basic

Code Snippet / Additional information

from __future__ import annotations

from typing import TypeVar


NUM_T = TypeVar('NUM_T', int, float)


def funcA(value: list[NUM_T | list[NUM_T]]):
    ...


def funcB(value: list[NUM_T | list[NUM_T]]):
    funcA(value)

Expected behaviour

No error.

Actual behaviour

Error shown on line 14:

Argument of type "list[NUM_T@func_B | list[NUM_T@func_B]]" cannot be assigned to parameter "value" of type "list[NUM_T@func_A | list[NUM_T@func_A]]" in function "func_A"
  TypeVar "_T@list" is invariant
    Type "NUM_T@func_B | list[NUM_T@func_B]" cannot be assigned to type "NUM_T@func_A | list[NUM_T@func_A]"
      Type "int* | float*" cannot be assigned to type "NUM_T@func_A | list[NUM_T@func_A]"
        Type "int*" cannot be assigned to type "NUM_T@func_A | list[NUM_T@func_A]"
          "int*" is incompatible with "list[NUM_T@func_A]"
        Type "float*" cannot be assigned to type "NUM_T@func_A | list[NUM_T@func_A]"
          "float*" is incompatible with "list[NUM_T@func_A]"
      Type "list[NUM_T@func_B]" cannot be assigned to type "NUM_T@func_A | list[NUM_T@func_A]"

[erictraut]

Why are you using a type variable in this case? Do you mean to use a union type?

NUM_T = Union[int, float]

or

T = TypeVar("T")
LIST_T = list[T | list[T]]
NUM_T = Union[LIST_T[int], LIST_T[float]]

[vishwa-raj]

@erictraut It's a fabricated example from a larger piece of code whose type annotation may look like list[NUM_T] | list[tuple[NUM_T, NUM_T]].
What we need here is to be able to allow both int and float type inputs.
If I just use float or float | int instead of NUM_T, I cannot pass an input like [1, 2, 3] without raising a type-annotation error as the input is of of type list[int] and list is invariant.
I can enumerate all combinations of int and float, like list[int] | list[float] | list[tuple[int, int]] | list[tuple[int, float]] ... but that will get unwieldy as the original type annotation includes many more combinations.

[erictraut]

This will be fixed in the next release. I've improved the heuristics for type matching when the source and destination types are both unions and invariance is enforced. The rules for matching in this case are not well defined and can be ambiguous in some cases where generics are involved in the destination type, but I've improved the heuristics to work in more cases including the one above.

[bschnurr]

This issue has been fixed in version 2021.7.3, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202173-14-july-2021