Empty dictionary argument is incompatible with union of mappings

[tobast]

Bug Report

When calling a function that calls for an argument of type Union[Mapping[x,z], Mapping[y,z]], the empty dictionary {} does not type-check.

I encountered this problem when trying to call subprocess.run with an empty environment, that is,

subprocess.run(..., env={})

To Reproduce

The smallest I've been able to reduce this issue to is the following code:

from typing import Optional, Union, Mapping
from os import PathLike


def foo(
    x: Union[
        Mapping[bytes, str],
        Mapping[str, str],
    ]
):
    pass


foo({})

Expected Behavior

I expect this code to type check, as the empty dictionary {} type checks against both sides of the Union — that is, if I remove the Union from the above code and leave only either side, mypy reports no issue.

Actual Behavior

Mypy reports an incompatible type:

test.py:14: error: Argument 1 to "foo" has incompatible type "Dict[<nothing>, <nothing>]"; expected "Union[Mapping[bytes, str], Mapping[str, str]]"  [arg-type]

If I break down the call to foo into

arg = {}
foo(arg)

mypy now complains that arg must be type annotated.

Your Environment

• Mypy version used: 1.0.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini: none
• Python version used: 3.10.9

[wxgeo]

I ran into the same bug today. Minimal example:

def f() -> dict[int, str] | dict[int, float]:
    return {}

error: Incompatible return value type (got "Dict[<nothing>, <nothing>]", expected "Union[Dict[int, str], Dict[int, float]]")

Note that using this in a simple affectation, like d: dict[int, str] | dict[int, float] = {}, will result in the same error message.