__new__ return types are not respected (reported as error and overwritten by the current class) unless the return type is a subclass of the current class #19624
Description
new.py:
from typing import cast
class A:
def __new__(cls) -> int:
return 1
class B:
def __new__(cls) -> "C":
return cast(C, None)
class C(B):
def __new__(cls) -> B:
return cast(B, None)
reveal_type(A()) # Expect int (builtin.int)
reveal_type(B()) # Expect C (new.C)
reveal_type(C()) # Expect B (new.B)Run
mypy new.py --config-file=(The --config-file= ignores configuration files)
Expected:
new.py:15: note: Revealed type is "builtin.int"
new.py:16: note: Revealed type is "new.C"
new.py:17: note: Revealed type is "new.B"
Actual:
new.py:4: error: Incompatible return type for "__new__" (returns "int", but must return a subtype of "A") [misc]
new.py:12: error: Incompatible return type for "__new__" (returns "B", but must return a subtype of "C") [misc]
new.py:15: note: Revealed type is "new.A"
new.py:16: note: Revealed type is "new.C"
new.py:17: note: Revealed type is "new.C"
Found 2 errors in 1 file (checked 1 source file)
Environment
- mypy 1.18.0+dev (compiled: no) (186515f)
python -m pip install -e . -C editable_mode=strict
- Python 3.13.2
Related issues
Same: #1020 #15182 #8330
Related: #14502 #12045 #9045
(Note: Some of these issues are noted as no longer reproducable in the comments. I didn't check their reproducability, but I am able to reproduce this one, which might mean a regression.)