Error on instance property and init-only variable with the same name in a dataclass

[mbraakhekke]

Bug Report

Consider the following class definition:

from dataclasses import dataclass, InitVar
@dataclass
class MyClass:
    prop: InitVar[int]
    _prop: int

    def __post_init__(self, prop: int):
        self._prop = prop

    @property
    def prop(self) -> int:
        return self._prop

Mypy reports an error on line 10: Name "prop" already defined on line 4.

Is this a false positive? If not, how can I avoid this short of renaming the "prop" attribute or getter method (or using #typing: ignore)?

Environment

• Mypy version used: 0.931
• Python version used: 3.10.1
• Operating system and version: Windows 10 x64

[A5rocks]

This is a problem with mypy's dataclasses plugin, I suppose. However, as mypy would have already processed the class (I think?) before the plugin gets to it, I'm not certain it can be fixed. I suppose the workaround would be to # type: ignore the necessary lines and run mypy with --warn-unused-ignores (you can stick this in a config file too) to tell you when you can remove the # type: ignore, ie because mypy gains support.

[mbraakhekke]

Thanks. I'll implement the suggest workaround.

[joshuafishman]

I also get "MyClass" has no attribute "prop" [attr-defined] in mypy for any reference of that attribute -- the workaround suggested doesn't address these.

[joshuafishman]

e.g.

from dataclasses import dataclass, InitVar


@dataclass
class Baz:
    foo: InitVar[int]

    @property  # type: ignore
    def foo(self) -> int:
        return 0


print(Baz(0).foo)

results in error: "Baz" has no attribute "foo" [attr-defined]

[ikonst]

Maybe if TypeInfo would start having a separate dict for annotations rather than put it all into names. Might be a bit of a refactor though, which could be less painful if TypeInfo.names becomes a "wrapper" around two maps, one with annotations and the other with identifiers.