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Type detection doesn't work with TypedDict #7344

Description

@priyank-p
  • Are you reporting a bug, or opening a feature request? Bug
  • Code that reproduces the issue:
from typing import Union, List, Iterable
from typing_extensions import Literal, TypedDict

# Goal here is to add mypy type so that
# if the dict's key is 'a' or 'b' mypy knows
# that the value should be int or str.
ABDictT = TypedDict('ABDictT', {
    'key': Literal['a', 'b'],
    'value': Union[str, int]
})

# If key is 'c', or 'd' then mypy should know
# value should be str, or int list.
CDDictT = TypedDict('CDDictT', {
    'key': Literal['c', 'd'],
    'value': Union[str, Iterable[int]],
})

ValueDictElementT = Union[ABDictT, CDDictT]
ValuesDictT = Iterable[ValueDictElementT]

def some_func(values: ValuesDictT) -> None:
    for element in values:
        if element['key'] == 'a':
            # The reveal_type here show the type to be
            # Union[str, int, Iterable[int]] since we
            # did a check that element['key'] is 'a' mypy
            # should be able to tell that the type is
            # ABDictT here?
            reveal_type(element['value'])
  • What is the actual behavior/output?
$ mypy --strict repro.py
repro.py:30: note: Revealed type is 'Union[builtins.str, builtins.int, typing.Iterable[builtins.int]]'
  • What is the behavior/output you expect?
    mypy should be able to tell that the type is ABDictT after checking that the key is a and therefore the output on the reveal_type should be Union[str, int].
$ mypy --strict repro.py
repro.py:30: note: Revealed type is 'Union[builtins.str, builtins.int]'
  • What are the versions of mypy and Python you are using? Python: 3.6.8, mypy: 0.720
    Do you see the same issue after installing mypy from Git master? Yes
  • What are the mypy flags you are using? (For example --strict-optional) I did use --strict but it can also be reproduced with no flags.

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