Mypy loses type of the TypedDict when unpacked

user_1348303 0
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I have the following code when trying to spread the dictionary from typing import TypedDict

class MyDict(TypedDict):
    foo: int

def test(inp: MyDict):
    m: MyDict = inp # OK
    n: MyDict = {**inp} # <-- ERROR

I receive an error Expression of type "dict[str, object]" cannot be assigned to declared type

Any idea how can I preserve the type after spread?

python-typing mypy python-3.x python
1 Answers

Currently, neither mypy nor pyright is smart enough to infer the type of an unpacked dict. See https://github.com/python/mypy/issues/4122.

As a workaround use typing.cast:

from typing import cast, TypedDict


class MyDict(TypedDict):
    foo: int


def test(inp: MyDict):
    m: MyDict = inp # OK
    n: MyDict = cast(MyDict, {**inp}) # OK
user_80910930
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