Mypy disallows overriding an attribute with a property

[rowillia]

For example:

class Foo:
 def __init__(self) -> None:
 self.x = 42

class Bar(Foo):
 def __init__(self) -> None:
 self.y = 9000

 @property
 def x(self) -> int:
 print('got x!!!')
 return self.y

 @x.setter
 def x(self, value: int) -> None:
 print('set x!!!')
 self.y = value

This should be allowed as it doesn't violate LSP as long as x implements both a getter and a setter of the correct type.

[ilevkivskyi]

Hm, interesting, this is already allowed for structural subtypes:

class Foo(Protocol):
 x: int

class Bar:
 def __init__(self) -> None:
 self.y = 9000

 @property
 def x(self) -> int:
 print('got x!!!')
 return self.y

 @x.setter
 def x(self, value: int) -> None:
 print('set x!!!')
 self.y = value

x: Foo = Bar() # OK

I think this is a bug, it should be allowed for nominal classes as well.

[JukkaL]

Yes, this is a bug. I was under the impression that this was already working.

[elazarg]

Technically it does violate LSP; there should be a warning about the missing @x.deleter.

[emmatyping]

Yes, but if you add a deleter it still complains.

[JelleZijlstra]

I don't think mypy should actually care about whether a deleter is present though, since deleting attributes is rare and if you do it your code is unlikely to be type-safe anyway.

[elazarg]

True, but overriding an attribute with a property is also not so common, so a reminder that it's not complete might be useful.

[srittau]

There is a different error message when using property() as a function:

class Base:
 prop: bool

class Sub(Base):
 def _get_prop(self) -> bool:
 return False
 def _set_prop(self, value: bool) -> None:
 pass
 prop = property(_get_prop, _set_prop)

foo.py:9: error: Incompatible types in assignment (expression has type "property", base class "Base" defined the type as "bool")

[erickpeirson]

In mypy==0.720, if I slightly modify the example in #4125 (comment) (above) based on the documentation's guidance that:

Explicitly including a protocol as a base class is also a way of documenting that your class implements a particular protocol, and it forces mypy to verify that your class implementation is actually compatible with the protocol.

...I get the following behavior:

class Foo(Protocol):
 x: int

class Bar(Foo):
 def __init__(self) -> None:
 self.y = 9000

 @property # Signature of "x" incompatible with supertype "Foo"
 def x(self) -> int: # Signature of "x" incompatible with supertype "Foo"
 print('got x!!!')
 return self.y

 @x.setter
 def x(self, value: int) -> None:
 print('set x!!!')
 self.y = value

x: Foo = Bar()

I would expect this to be OK for the same reasons that the original protocol example (no inheritance) is OK.

[ilevkivskyi]

Setting priority to high, see duplicate issue #6759 (comment) for motivation.