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## Summary If you manage to create an `typing.GenericAlias` instance without us knowing how that was created, then we don't know what to do with this in a type annotation. So it's better to be explicit and show an error instead of failing silently with a `@Todo` type. ## Test Plan * New Markdown tests * Zero ecosystem impact
1021 B
1021 B
GenericAlias in type expressions
We recognize if a types.GenericAlias instance is created by specializing a generic class. We don't
explicitly mention it in our type display, but list[int] in the example below is a GenericAlias
instance at runtime:
Numbers = list[int]
# At runtime, `Numbers` is an instance of `types.GenericAlias`. Showing
# this as `list[int]` is more helpful, though:
reveal_type(Numbers) # revealed: <class 'list[int]'>
def _(numbers: Numbers) -> None:
reveal_type(numbers) # revealed: list[int]
It is also valid to create GenericAlias instances manually:
from types import GenericAlias
Strings = GenericAlias(list, (str,))
reveal_type(Strings) # revealed: GenericAlias
However, using such a GenericAlias instance in a type expression is currently not supported:
# error: [invalid-type-form] "Variable of type `GenericAlias` is not allowed in a type expression"
def _(strings: Strings) -> None:
reveal_type(strings) # revealed: Unknown