in keyword should narrow Union[TypedDict, ...] #9953
Description
Feature
Using in on a Union[TypedDict] should narrow the Union. There is already something similar for isinstance that can narrow based on types. If a certain key exists you could narrow down that your types from a Union as well. This would prevent an error saying a certain key is missing because you checked already, they code can safely access the key. Currently the only workaround for this would be to use cast() but really shouldn't be necessary.
Pitch
After the change the following code should pass validation.
from typing import castm Union
from typing_extensions import TypedDict
class Movie(TypedDict):
name: str
year: int
class MovieResponse(TypedDict):
movie: Movie
class Book(TypedDict):
name: str
year: int
class BookResponse(TypedDict):
book: Book
MediaResponse = Union[MovieResponse, BookResponse]
response : MediaResponse = MovieResponse(movie=Movie(name="Blade Runner", year=1982))
if 'movie' in response:
reveal_type(response) # Current: Union[MovieResponse, BookResponse]
# Desired: Union[MovieResponse]
print(response["movie"]) # This fails due to `BookResponse` not having a key "movie"
# but we already validated existance of the 'movie' key and that type
# should no longer be considered here
print(cast(MovieResponse, response)["movie"]) # current workaround
Activity
michaeloliverx commentedon May 23, 2023
You can get around this by giving each typeddict a unique tag but it would be nice not to have to do this. TypeScript has this feature "The
inoperator narrowing"choucavalier commentedon Jul 17, 2023
indeed this would be great! having mypy errors i should not have because of this
ikonst commentedon Jul 17, 2023
Since #13838 this already works, but you have to decorate the TypedDicts with
@final:since otherwise you can't rule out
@AlexWaygood we could close this issue, but I do think we could better document it at the very least (#15695).
p.s. I also wonder if mypy should surface a hint, e.g.
choucavalier commentedon Jul 20, 2023
Thanks @ikonst indeed this worked!
ikonst commentedon Jul 20, 2023
FWIW we're considering removing the need to add
@final: #15697