No type inference for dicts in union

[ikonst]

from typing import *

def foo() -> Dict[str, Any]:
    return {}

def bar() -> Union[Dict[str, Any], Dict[int, Any]]:
    return {}

In foo, {} is inferred as Dict[str, Any].
In bar, I get:

error: Incompatible return value type (got "Dict[<nothing>, <nothing>]", expected "Union[Dict[str, Any], Dict[int, Any]]")

[gvanrossum]

I’m sure the cause is the union, not the tuple.

[ikonst]

@gvanrossum thanks, you're right - corrected the issue title and the sample code

[oraluben]

@ikonst you could try this to have a workaround, though the semantics has changed

def bar() -> Dict[Union[str, int], Any]:
    return {}

[ilevkivskyi]

This may be a duplicate of #2164

[ikonst]

@oraluben - the actual code is more complex:

from typing import *

from flask import Response

JsonDict = Dict[str, Any]

_FlaskResponse = Union[
    str,
    bytes,
    Callable,
    Response,
]
_FlaskRestfulResponse = Union[
    JsonDict,
    _FlaskResponse,
]
FlaskRestfulMethodReturnType = Union[
    _FlaskRestfulResponse,
    Tuple[Optional[_FlaskRestfulResponse], int],  # (response, status_code)
    Tuple[Optional[_FlaskRestfulResponse], int, Dict[str, str]],  # (response, status_code, headers)
    Tuple[Optional[_FlaskRestfulResponse], Dict[str, str]],  # (response, headers)
]

and then this works:

class MyResource(Resource):
   def get() -> FlaskRestfulMethodReturnType:
       return {}

and this works:

class MyResource(Resource):
   def get() -> FlaskRestfulMethodReturnType:
       return {"foo": "bar"}, 200

but this doesn't:

class MyResource(Resource):
   def get() -> FlaskRestfulMethodReturnType:
       return {}, 200

[JukkaL]

This happens with other invariant containers as well, such as lists. I just encountered an instance where mypy couldn't infer the type of a {} function argument when calling string.Template.substitute, since the type of the argument in the signature is a union.

[ruuda]

This also happens with dicts inside a tuple:

from typing import Dict, Tuple, Union

T = Dict[str, str]
U = Tuple[T]

def f() -> U:
    return {},

def g() -> Union[U, U]:
    return {},

10: error: Incompatible return value type (got "Tuple[Dict[<nothing>, <nothing>]]", expected "Union[Tuple[Dict[str, str]], Tuple[Dict[str, str]]]")