False-positive error on overriding attribute of base class with rtype-annotated property

[sizmailov]

(bug report)

Annotation of property of derived class with -> Any makes mypy complain about incompatibility with base class. The exact rtype doesn't matter, the error is still the same. The MRE is a simplified version of stubgen output.

MRE:

from typing import Any

class A:
    doc = ""

class B(A):
    # remove "-> Any" to make mypy happy
    @property
    def doc(self) -> Any:
        return ""

Expected: no errors reported

Actual:

foo.py:11: error: Signature of "doc" incompatible with supertype "A"
Found 1 error in 1 file (checked 1 source file)

command line command: mypy foo.py

version info:
mypy: 0.790+dev.ffdbeb3d47ccb2d9691b36993be0a18e0718d997
python: 3.7.3

P.S.: The error message of Signature of "doc" incompatible with supertype "A" should be more descriptive. It's good enough for simple cases, but rather cryptic for more involved examples. (I realize it might be not so trivial to propagate the root source of incompatibility and represent it nicely)

[JukkaL]

The error message is kind of reasonable, since doc is mutable in A but not in B. If you don't have an annotation, mypy treats the code as untyped and omits various checks.

The error message of Signature of "doc" incompatible with supertype "A" should be more descriptive

I think that mypy sometimes produces a better message, but not in this case. Would you like to create a separate issue about the better error message?

[sizmailov]

The error message is kind of reasonable, since doc is mutable in A but not in B.

If it so, the next snippet should pass the check, right?

from typing import Any


class A:
    doc = ""


class B(A):
    @property
    def doc(self) -> Any:
        return ""

    @doc.setter
    def doc(self, value: str) -> None:
        print("set")

    @doc.deleter
    def doc(self) -> None:
        print("del")

In fact mypy reports the same error:

example.py:9: error: Signature of "doc" incompatible with supertype "A"
example.py:10: error: Signature of "doc" incompatible with supertype "A"
Found 2 errors in 1 file (checked 1 source file)

At the moment I'm not sure what makes mypy complain about this snippet to file another issue.

[sizmailov]

Apparently @property decorator is treated as black box and effectively () -> Any gets compared to str.

Another form of overriding value-attribute with property triggers another false-positive error:

class B(A):
    doc = property(...)
# Incompatible types in assignment (expression has type "property", base class "A" defined the type as "str")

Same applies to custom descriptors as well:

class B(A):
    doc = RevealAccess(10, 'var "x"')
# Incompatible types in assignment (expression has type "RevealAccess", base class "A" defined the type as "str")

[ilevkivskyi]

Now mypy gives a nice error

test.py:8: error: Cannot override writeable attribute with read-only property  [override]

and passes when setter is added.