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
Feature
Using
inon aUnion[TypedDict]should narrow theUnion. There is already something similar forisinstancethat 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 usecast()but really shouldn't be necessary.Pitch
After the change the following code should pass validation.