Misleading type inference on complicated dict literal

[alanhdu]

This might not be a "bug" per se, but I'm going to report it anyways in case.

Running mypy 0.761 on Python 3.6 on the following code

def get_api_message(a: str, b: str, c: int, d: str) -> None:
    api_message = {
        "d": d,
        "api_request": {"c": c, "start_stream_request": {"b": b, a: {}}},
    }
    reveal_type(api_message)

gives us a revealed type of:

$ mypy scratch.py
Revealed type is 'builtins.dict[builtins.str*, typing.Collection*[builtins.str]]'

This led to a confusing error downstream where code that edits part of the dict literal gets a Value of type "Collection[str]" is not indexable.

I get why Collection is being inferred, but probably in this case it'd be better to either explicitly require a type annotation or to infer Dict[str, Any] instead.

[JukkaL]

Yeah, the inferred type is confusing, but I'm not sure if there's anything we can easily do about it, since it's consistent with how mypy does type inference in general. One of the underlying problems is that string is a sequence of strings, which is a feature of Python (even though it sometimes feels like a misfeature).

[yesthesoup]

For anyone else ending up here, I solved this by declaring my dictionary as a JsonDict type

i.e. given the example in the OP

from mypy.types import JsonDict

api_message: JsonDict = { .... }

[JelleZijlstra]

You should not be importing mypy.types.JsonDict or anything else from mypy (unless you're writing a mypy plugin). Mypy is not meant to be used as a library and its internal types may change at any time. mypy.types.JsonDict is currently an alias for Dict[str, Any], so if that's the type you want, write Dict[str, Any] directly.

[yesthesoup]

You should not be importing mypy.types.JsonDict or anything else from mypy (unless you're writing a mypy plugin). Mypy is not meant to be used as a library and its internal types may change at any time. mypy.types.JsonDict is currently an alias for Dict[str, Any], so if that's the type you want, write Dict[str, Any] directly.

Thanks for the feedback! I wasn't aware