Pyright handling items method of TypedDict

[mheguy]

Heya, the behaviour of the following is unexpected:

from typing import TypedDict

class TestClass(TypedDict):
    input: str

for _, bloop in TestClass(input="input").items():
    _.lower()
    bloop.lower() # error: Cannot access attribute "lower" for class "object" - Attribute "lower" is unknown (reportAttributeAccessIssue)

This doesn't seem like a bug in Pyright, it's just doing what the type hints say.
The typeshed says: def items(self) -> dict_items[str, object]: ...

Is there a (reasonable) way to work around this?

[erictraut]

Yeah, pyright is working correctly here. The way TypedDict is defined in the typing spec, it is a structural type, and a value that is compatible with a TypedDict can contain additional items beyond what that TypedDict defines. That means when iterating over all items, a type checker needs to assume there are potentially additional items present whose values are object type.

If you know that there are no additional items present, you can add an assert isinstance(bloop, str) statement in your loop.

You may also be interested in draft PEP 728, which proposes to add support for "closed" TypedDict definitions. This PEP is in flux, but pyright has provisional support for the latest draft if you would like to play with it. You need to set enableExperimentalFeatures to true.