Casting from a TypedDict to a Mapping can be a bit more lax

[tusharsadhwani]

Currently, this code fails type checking:

from collections.abc import Mapping
from typing import TypedDict

def my_func(v: Mapping[str, str]):
    print(v.items())

class MyTypedDict(TypedDict):
    foo: str

val: MyTypedDict = {"foo": "a"}
my_func(val)

Changing the annotation of v to Mapping[str, object] works. But since the TypedDict only contains str fields, I think this cast should work? Is there a reason for the cast to not work right now?

[erictraut]

The typing spec specifically disallows assignment to a more specific Mapping type because this is not type safe. You can read about this in the official typing spec here. A TypedDict is an "open" structural type, and a compatible dictionary is allowed to contain keys that are not specified in the TypedDict definition.

There is a draft PEP that defines support for "closed" TypedDicts. This PEP hasn't yet been approved, but pyright contains provisional support for the latest draft. You can try it by setting enableExperimentalFeatures to true in your pyright configuration.