Closed
Description
Bug report
Bug description:
I realized that the type_params parameter of TypeAliasType behaves differently when it is omitted vs passing an empty tuple (), in the latter case the resulting instance is always subscriptable. I doubt this is an undocumented feature.
from typing import List, TypeAliasType, TypeVar
# < Errors like expected >
Simple = TypeAliasType("Simple", int)
try:
Simple[int]
except TypeError:
print("Expected: Simple is not subcriptable")
# Not allowed assignment
T = TypeVar('T')
MissingTypeParamsErr = TypeAliasType("MissingTypeParamsErr", List[T])
assert MissingTypeParamsErr.__type_params__ == ()
assert MissingTypeParamsErr.__parameters__ == ()
try:
MissingTypeParamsErr[int]
except TypeError:
print("Expected: Simple is not subcriptable")
# < unexpected cases >
MissingTypeParams = TypeAliasType("MissingTypeParams", List[T], type_params=())
assert MissingTypeParams.__type_params__ == ()
assert MissingTypeParams.__parameters__ == ()
# These cases do not raise an error
MissingTypeParams[int]
MissingTypeParams[[]]
MissingTypeParams[()]
# This also do not raise an error
Simple2 = TypeAliasType("Simple2", int, type_params=())
assert Simple2 .__type_params__ == ()
assert Simple2 .__parameters__ == ()
Simple2[int]
Simple2[[]]
Simple2[()]CPython versions tested on:
3.13
Operating systems tested on:
Linux