TypedDict has no key when checking for key

[Dreamsorcerer]

Minimal test case:

[case testTaggedUnionByKey]
from mypy_extensions import TypedDict
from typing import Union
class A(TypedDict):
    foo: int
class B(TypedDict):
    bar: str
def a(a: Union[A, B]):
    return a["foo"] if "foo" in a else None
[builtins fixtures/tuple.pyi]

The code is checking if the key is present, so this should pass correctly, but instead fails with error: TypedDict "B" has no key 'foo'.

[Dreamsorcerer]

Coming back to this, it looks like this should be supported by the tagged union pattern documented here:
https://mypy.readthedocs.io/en/stable/literal_types.html#tagged-unions

That documentation uses different Literal values on a key, but we should be able to achieve the same thing by testing whether a key exists or not.

[Dreamsorcerer]

Curiously, I can't even get the documented tagged union functionality to work in a test case, or find any code related to the feature. For example, this still fails:

[case testTaggedUnion]
from mypy_extensions import TypedDict
from typing import Union
from typing_extensions import Literal
class A(TypedDict):
    foo: Literal[2]
class B(TypedDict):
    foo: Literal[3]
    bar: str
def a(a: Union[A, B]):
    return a["bar"] if a["foo"] == 3 else None
[builtins fixtures/tuple.pyi]

[hauntsaninja]

Your second example passes for me when I run mypy on it as a file (didn't try as a test); maybe you need a different fixture or something. You might be interested in #8151

[Dreamsorcerer]

Thanks for digging that up, must just have been something wrong with the way I was running the test. I'm sure I've used the feature in my code before.

[AlexWaygood]

I'm closing this issue, as the root cause of the mypy error here is discussed more precisely in #9953