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Function returning NotImplementedType cannot return NotImplemented if strict=True #18914
Description
Bug Report
A function returning NotImplementedType cannot return NotImplemented if strict=True.
(A clear and concise description of what the bug is.)
To Reproduce
from types import NotImplementedType
def foo() -> NotImplementedType:
return NotImplementedExpected Behavior
Type checks even with --strict
Actual Behavior
main.py:4: error: Returning Any from function declared to return "_NotImplementedType" [no-any-return]
Found 1 error in 1 file (checked 1 source file)
Your Environment
- Mypy version used: 1.15.0
- Mypy command-line flags:
--strict - Mypy configuration options from
mypy.ini(and other config files): None - Python version used: 3.12
Reacted by wyattscarpenter, Glyph and Matt Bogosian
Activity
sterliakov commentedon Apr 12, 2025
Ohh.
NotImplementedis really underspecified in typing context. For all comparison methods we silently assume that it can be returned from bool-returning methods like__lt__. I agree thatNotImplementedshouldn't be treated asAnyeverywhere, but this change will be rather difficult to release - everything from stdlib to third-party stubs relies on it.However, it's certainly reasonable to make
NotImplementeda better exception - perhaps by tweaking itsTypeOfAnyto stop it from showing in the reports.