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mypy confusion with empty dict as a Mapping when within a union #16208
Description
I've created a file.py with the following content,
from typing import Any, Mapping
a: tuple[Mapping[str, Any], dict] | tuple[Mapping[str, Any], int] = {}, 200
b: tuple[Mapping[str, Any], int] = {}, 200 Then installed mypy (Python 3.11.1),
$ pip install mypy
Collecting mypy
Using cached mypy-1.5.1-cp311-cp311-macosx_10_9_x86_64.whl (10.5 MB)
Collecting typing-extensions>=4.1.0
Using cached typing_extensions-4.8.0-py3-none-any.whl (31 kB)
Collecting mypy-extensions>=1.0.0
Using cached mypy_extensions-1.0.0-py3-none-any.whl (4.7 kB)
Installing collected packages: typing-extensions, mypy-extensions, mypy
Successfully installed mypy-1.5.1 mypy-extensions-1.0.0 typing-extensions-4.8.0
Which gives this error,
mypy file.py
file.py:3: error: Incompatible types in assignment (expression has type "tuple[dict[<nothing>, <nothing>], int]", variable has type "tuple[Mapping[str, Any], dict[Any, Any]] | tuple[Mapping[str, Any], int]") [assignment]
Found 1 error in 1 file (checked 1 source file)
Which is odd as line 4 (the b assignment) is fine.
Note this relates to a typing issue in Flask.
Activity
AlexWaygood commentedon Oct 1, 2023
This bug is reproducible with mypy 1.5.1, but has already been fixed on the master branch: https://mypy-play.net/?mypy=master&python=3.11&gist=2f5367d2a6b23d57ad81f2aabbcbe5ea. It was likely fixed by #16122 (but I haven't checked that).
(The fix probably won't be included in mypy 1.6, unfortunately; you'll probably have to wait for mypy 1.7.)
pgjones commentedon Oct 1, 2023
Thanks, great to hear it has been fixed.