No type inference for dicts in union

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]]")

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

@gvanrossum

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

@ikonst

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

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

This may be a duplicate of #2164

@oraluben

@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

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.

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]]]")

You can work around this issue with:

from typing import *

def get_empty_list_str() -> List[str]:
    return []

def get_empty_list_int() -> List[int]:
    return []

foo: Union[List[str], List[int]] = get_empty_list_str()  # fine
bar: Union[List[str], List[int]] = get_empty_list_int()  # fine
baz: Union[List[str], List[int]] = []  # error

i.e., create a function to create an empty container with a defined type.
You can solve the original issue like this (note, it doesn't matter what the type of your constructor function is as long as it is one of the Union types):

from typing import *

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

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

Workaround python/mypy#6463

This false-positive is also there with unions of lists when assigning an empty list (e.g. var: Union[list[int], list[str]] = [].

The issue is a few years old at this point but the case is not obscure enough that it would only occur in very few codebases. What's the status on this?

@JosuaKrause

Thanks @JosuaKrause for the work around. This actually can be done very concisely in newer python versions by using list[str](), dict[str, int](), etc. as constructors for a typed empty iterators:

foo: list[str] | list[int] = list[str]()  # fine
bar: list[str] | list[int] = list[int]()  # fine
baz:  list[str] | list[int] = []  # error

Special-case type inference of empty collections (#16122)

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No type inference for dicts in union #6463

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No type inference for dicts in union #6463

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gvanrossum commented on Feb 22, 2019

ikonst commented on Feb 22, 2019

oraluben commented on Feb 25, 2019

ilevkivskyi commented on Feb 25, 2019

ikonst commented on Feb 26, 2019

JukkaL commented on Mar 29, 2019

ruuda commented on Jun 5, 2019

JosuaKrause commented on Apr 2, 2021

berzi commented on May 24, 2022

klieret commented on Oct 26, 2022

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