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Subclasses cannot use properties to override super-class attributes #6759
Description
When subclassing, overriding a class attribute with a property with the same type causes type checking failures. This is (I think?) a fairly common way for subclasses to introduce dynamic behaviour transparently to consumers, so my expectation is that this would not cause failures.
Python Code
from typing import List
class Super:
x: List[str] = []
reveal_type(x)
class Sub(Super):
@property
def x(self) -> List[str]:
return []
reveal_type(x)mypy Output
$ mypy reproducer.py
reproducer.py:7: error: Revealed type is 'builtins.list[builtins.str]'
reproducer.py:13: error: Signature of "x" incompatible with supertype "Super"
reproducer.py:15: error: Revealed type is 'def (self: reproducer.Sub) -> builtins.list[builtins.str]'
Versions
$ python3 --version # This is the packaged version in Ubuntu 19.04
Python 3.7.2+
$ mypy --version
mypy 0.710+dev.b29a4330d2d7d1032c30a1990e7f1c477db070fd
Activity
JelleZijlstra commentedon May 3, 2019
This is unsafe because the superclass allows writing to
x(y = Super(); y.x = []works), but the subclass doesn't (y = Sub(); y.x = []is an AttributeError).OddBloke commentedon May 3, 2019
Aha, right, that makes sense. Thanks!
ilevkivskyi commentedon Aug 15, 2019
Mypy however still complains if I make the property settable. I looked at the code and it is indeed buggy (several corner cases with
is_propertyandis_settable_propertyare not considered).Generally, the way how we check incompatibility with supertype is fragile and uses a lot of code duplication with
checkmember.py. I would say we should simplify this significantly by just re-usingcheckmember.py. See also #5136ilevkivskyi commentedon Aug 16, 2019
Oh, this is actually a duplicate of #4125