If Python classes do not define __bool__ or __len__, bool(<class object>) defaults to True. However, mypy (tested with: v1.15.0) doesn't seem to consider this. class A: def __init__(self) -> None: self._value = 42 def foobar(self) -> int: return self._value def foobar(a: A | None) -> int | None: # case: `a is None` ⇒ returns `None` # case: `a is not None` ⇒ returns `42` # however: mypy complains... # `Incompatible return value type (got A | int | None), expected "int | None"` return a and a.foobar() To my understanding, the expression a and a.foobar() can only evaluate to type A if a is not None and bool(a) == False, which cannot happen. My conclusion is that mypy doesn't consider that a is not None ⇒ bool(a) == True. Am I right that mypy simply fails on this? Is there any convenient workaround to write optional_obj and optional_obj.attribute in a mypy-compatible way? Continue reading...
