"'field' in typed_dict_union" discrimination

Feature

Similar to tagged unions, I wish an in check could discriminate the following:

class A(TypedDict)
   foo: int

class B(TypedDict):
   bar: int

union = Union[A, B]
value: int
if 'foo' in union:
  value = union['foo']
else:
  value = union['bar']

Similarly:

value: int
if foo := union.get('foo'):
  value = foo
else:
  value = union['bar']

This has been discussed previously in various typing forums. (I can't find the threads currently, but perhaps someone else can find a pointer.)

The problem here is that narrowing based on field discrimination within a TypedDict isn't type safe unless that TypedDict class is marked @final. Otherwise it would be possible for a subclass of B to add a field foo.

With an @final decorator on A and B it should be possible for mypy to implement type narrowing based on field discrimination. I've implemented this support in pyright. Currently, mypy emits an error if you attempt to add a @final decorator to a TypedDict class.

from typing import TypedDict, final

@final
class A(TypedDict):
    foo: int

@final
class B(TypedDict):
    bar: int

def func(union: A | B):
    if "foo" in union:
        reveal_type(union)  # A
    else:
        reveal_type(union)  # B

#7981 is probably the right mypy issue to track this

@hauntsaninja

Thanks for the pointer @hauntsaninja, indeed appears to be addressed in #13557. Happy to test once it makes its way into the next release.

Only half the feature is implemented; want to take a shot at completing it?

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"'field' in typed_dict_union" discrimination #13802

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"'field' in typed_dict_union" discrimination #13802

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erictraut commented on Oct 3, 2022

hauntsaninja commented on Oct 3, 2022

ikonst commented on Oct 4, 2022

hauntsaninja commented on Oct 4, 2022

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