type hint with "Union" operator doesn't work properly

[djccnt15]

Environment data

• Language Server version: 2022.11.20 (pyright 470188d0)
• OS and version: windows 11
• Python version (& distribution if applicable, e.g. Anaconda): 3.10, 3.11

Code Snippet

I made simple function which returns identity matrix like below

vector = list[float | int]
matrix = list[vector]

def mat_identity(n: int) -> matrix:
    """
    returns n by n sized identity matrix
    """

    I = [[1 if i == j else 0 for j in range(n)] for i in range(n)]
    return I

Repro Steps

just type the code

Expected behavior

obviously the function shouldn't have any problem, as code below returns no error at all

vector = list[float | int]
matrix = list[vector]

m: matrix = [[1, 0, 0], [0, 1, 0], [0, 0, 1]]

Actual behavior

pylance warn error like below

Expression of type "list[list[int]]" cannot be assigned to return type "matrix"
  "list[list[int]]" is incompatible with "matrix"
    TypeVar "_T@list" is invariant
      "list[int]" is incompatible with "vector"
        TypeVar "_T@list" is invariant
          Type "int" cannot be assigned to type "float | int"

[erictraut]

Yes, this behavior is correct. To understand why, please review this documentation about static type concepts.

There are two simple options for eliminating the type violation error:

1. Do not assign the intermediate value to variable I. Move the return keyword directly before the list comprehension. This allows pylance to use bidirectional type inference to infer the correct return type.
2. Supply a type declaration for intermediate variable I (I: matrix). This once again allows pylance to use bidirectional type inference to determine the intended type.