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TypeVar("T") can be anything, unless a type guard hints at something else #16742
Closed as not planned
Description
Bug Report
According to the docs, a TypeVar("T") can be anything, but type guards hint at something.
To Reproduce
from typing import TypeVar
PT = TypeVar("PT")
def f(p: PT) -> PT:
reveal_type(p) # Revealed type is "PT`-1"
if isinstance(p, bool):
reveal_type(p) # Revealed type is "builtins.bool"
return p
if isinstance(p, int):
reveal_type(p) # Revealed type is "builtins.int"
return p
reveal_type(p) # Revealed type is "PT`-1"
return p
f(False) # Incompatible return value type (got "bool", expected "PT")
f(0) # Incompatible return value type (got "int", expected "PT")
f(.0)See https://gist.github.com/mypy-play/72f6621d0539aae2d7a3483ce389919d
Expected Behavior
I would expect the above example to pass. But then again, I've opened tickets in the past which proved to be either my own misunderstanding or my own misuse :)
Actual Behavior
main.py:8: note: Revealed type is "PT`-1"
main.py:11: note: Revealed type is "builtins.bool"
main.py:12: error: Incompatible return value type (got "bool", expected "PT") [return-value]
main.py:15: note: Revealed type is "builtins.int"
main.py:16: error: Incompatible return value type (got "int", expected "PT") [return-value]
main.py:18: note: Revealed type is "PT`-1"
Found 2 errors in 1 file (checked 1 source file)
Activity
[-]`TypeVar("T")` can be anything, unless a type guard[/-][+]`TypeVar("T")` can be anything, unless a type guard hints at something else[/+]hauntsaninja commentedon Jan 4, 2024
Thanks for the issue, I think this is a duplicate of #15151. See also #15711 / #15377
alexei commentedon Jan 4, 2024
If a fix isn't planned, then what is the recommended workaround? Cast to
T?hauntsaninja commentedon Jan 4, 2024
Yes, type ignore or cast will work. Definitely a bug so will be fixed
alexei commentedon Jan 4, 2024
Ok, thanks