Safe conversion from TypedDict to Mapping not accepted

[dmoisset]

Bug Report

Assignment of a TypedDict into a Mapping with a wider type is not allowed despite being safe.

To Reproduce

from typing import TypedDict, Mapping

class TD(TypedDict):
    a: str
    b: int

type Data = Mapping[str, str | int]

value = TD(a="hello", b=123)

x: Data = value

Playground link

Expected Behavior

The assignment should be accepted. Every value of TD is a valid instance of Data, and being a mapping Data does not allow mutation.

Actual Behavior

main.py:11: error: Incompatible types in assignment (expression has type "TD", variable has type "Mapping[str, str | int]")  [assignment]
Found 1 error in 1 file (checked 1 source file)

Your Environment

Running on python 3.13, latest mypy (1.14.1), default settings.

[erictraut]

Mypy is correct to generate an error here. For an explanation, refer to this section of the Python typing spec. This isn't safe because an object can be compatible with a TypedDict and have additional items that are not defined in that TypedDict. These additional items must be assumed to have a value type of object.

There is a new draft PEP that adds the concept of a "closed" TypedDict, but mypy doesn't currently have support for this functionality.

[dmoisset]

Oh, I missed this scenario, thanks for the quick answers and the link to the (super clear) docs. A closed typed dict would help, for now I guess I will just cast my way out. Feel free to close this issue.