Recognize valid pattern of dataclasses with @property

[Cs4System]

I tried a pattern which is absolutely fine in runtime and should be valid code, but pylance is complaining a lot:

from dataclasses import InitVar, dataclass, field


@dataclass
class Animal:
    name: str

    def alive(self) -> str:
        return f"{self.name} is alive"


@dataclass
class Duck(Animal):
    age: InitVar[int]
    _age: int = field(init=False, repr=False)  # do not initialize, only register datafield

    def __post_init__(self, age: int):
        self.age = age  # uses setter!

    @property
    def age(self) -> int:
        return self._age

    @age.setter
    def age(self, value: int):
        if value < 0:
            raise ValueError("Age cannot be negative!")
        self._age = value

    def fly(self) -> str:
        return f"{self.name} flies"

    def swim(self) -> str:
        return f"{self.name} swims"


def main() -> None:
    duck = Duck(name="Daisy", age=2)
    print(duck.alive())
    print(duck.fly())
    print(duck.swim())
    print(duck.age)


if __name__ == "__main__":
    main()

problem detected by pylance:

[{
	"resource": "/d:/dev/python_oop/src/python_oop/1_4_simple_inherritance_init_dataclass.py",
	"owner": "Pylance5",
	"code": {
		"value": "reportRedeclaration",
		"target": {
			"$mid": 1,
			"path": "/microsoft/pylance-release/blob/main/docs/diagnostics/reportRedeclaration.md",
			"scheme": "https",
			"authority": "github.com"
		}
	},
	"severity": 8,
	"message": "Declaration \"age\" is obscured by a declaration of the same name",
	"source": "Pylance",
	"startLineNumber": 23,
	"startColumn": 5,
	"endLineNumber": 23,
	"endColumn": 8,
	"relatedInformation": [
		{
			"startLineNumber": 34,
			"startColumn": 9,
			"endLineNumber": 34,
			"endColumn": 12,
			"message": "See method declaration",
			"resource": "/d:/dev/python_oop/src/python_oop/1_4_simple_inherritance_init_dataclass.py"
		}
	],
	"modelVersionId": 565,
	"origin": "extHost1"
},{
	"resource": "/d:/dev/python_oop/src/python_oop/1_4_simple_inherritance_init_dataclass.py",
	"owner": "Pylance5",
	"code": {
		"value": "reportGeneralTypeIssues",
		"target": {
			"$mid": 1,
			"path": "/microsoft/pylance-release/blob/main/docs/diagnostics/reportGeneralTypeIssues.md",
			"scheme": "https",
			"authority": "github.com"
		}
	},
	"severity": 8,
	"message": "Dataclass __post_init__ method parameter type mismatch for field \"age\"\n  \"property\" is not assignable to \"int\"",
	"source": "Pylance",
	"startLineNumber": 26,
	"startColumn": 34,
	"endLineNumber": 26,
	"endColumn": 37,
	"relatedInformation": [
		{
			"startLineNumber": 23,
			"startColumn": 5,
			"endLineNumber": 23,
			"endColumn": 8,
			"message": "Field declaration",
			"resource": "/d:/dev/python_oop/src/python_oop/1_4_simple_inherritance_init_dataclass.py"
		}
	],
	"modelVersionId": 565,
	"origin": "extHost1"
},{
	"resource": "/d:/dev/python_oop/src/python_oop/1_4_simple_inherritance_init_dataclass.py",
	"owner": "Pylance5",
	"code": {
		"value": "reportAttributeAccessIssue",
		"target": {
			"$mid": 1,
			"path": "/microsoft/pylance-release/blob/main/docs/diagnostics/reportAttributeAccessIssue.md",
			"scheme": "https",
			"authority": "github.com"
		}
	},
	"severity": 8,
	"message": "Cannot assign to attribute \"age\" for class \"Duck*\"\n  \"age\" is an init-only field",
	"source": "Pylance",
	"startLineNumber": 27,
	"startColumn": 14,
	"endLineNumber": 27,
	"endColumn": 17,
	"modelVersionId": 565,
	"origin": "extHost1"
},{
	"resource": "/d:/dev/python_oop/src/python_oop/1_4_simple_inherritance_init_dataclass.py",
	"owner": "Pylance5",
	"code": {
		"value": "reportUnknownMemberType",
		"target": {
			"$mid": 1,
			"path": "/microsoft/pylance-release/blob/main/docs/diagnostics/reportUnknownMemberType.md",
			"scheme": "https",
			"authority": "github.com"
		}
	},
	"severity": 8,
	"message": "Type of \"age\" is unknown",
	"source": "Pylance",
	"startLineNumber": 51,
	"startColumn": 11,
	"endLineNumber": 51,
	"endColumn": 19,
	"modelVersionId": 565,
	"origin": "extHost1"
},{
	"resource": "/d:/dev/python_oop/src/python_oop/1_4_simple_inherritance_init_dataclass.py",
	"owner": "Pylance5",
	"code": {
		"value": "reportUnknownArgumentType",
		"target": {
			"$mid": 1,
			"path": "/microsoft/pylance-release/blob/main/docs/diagnostics/reportUnknownArgumentType.md",
			"scheme": "https",
			"authority": "github.com"
		}
	},
	"severity": 8,
	"message": "Argument type is unknown\n  Argument corresponds to parameter \"values\" in function \"print\"",
	"source": "Pylance",
	"startLineNumber": 51,
	"startColumn": 11,
	"endLineNumber": 51,
	"endColumn": 19,
	"modelVersionId": 565,
	"origin": "extHost1"
},{
	"resource": "/d:/dev/python_oop/src/python_oop/1_4_simple_inherritance_init_dataclass.py",
	"owner": "Pylance5",
	"code": {
		"value": "reportAttributeAccessIssue",
		"target": {
			"$mid": 1,
			"path": "/microsoft/pylance-release/blob/main/docs/diagnostics/reportAttributeAccessIssue.md",
			"scheme": "https",
			"authority": "github.com"
		}
	},
	"severity": 8,
	"message": "Cannot access attribute \"age\" for class \"Duck\"\n  \"age\" is an init-only field",
	"source": "Pylance",
	"startLineNumber": 51,
	"startColumn": 16,
	"endLineNumber": 51,
	"endColumn": 19,
	"modelVersionId": 565,
	"origin": "extHost1"
}]

This pattern should be recognized and have no type errors.
Thanks!

[github-actions]

Hi @Cs4System, I'm an AI Support assistant here to help with your issue. While the team reviews your request, I wanted to provide some possible tips and documentation that might help you in the meantime.

---

Similar Issues (FYI only, not solutions)

The following issues may or may not be related to your report. They are provided for reference only, as they may describe similar symptoms, affect the same feature, or address related concerns, but may not be the same issue.

• Dataclass unpacking and excessive type narrowing. Pylance reports mismatched types when using **data to initialize a dataclass. Pylance and type checking for **destructuring at fn call pylance-release#2443
• Protocol property vs. attribute type mismatch. Pylance flags “property” as incompatible with a plain attribute or vice versa in protocol definitions. Property/attribute in a protocol #1368

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[erictraut]

Pyright is working as designed here, so this isn't a bug.

You're redefining the class-scoped symbol age here, which isn't allowed by static type checkers. The two definitions for the symbol have conflicting types. That's not allowed by pyright (or other static type checkers).

[Cs4System]

@erictraut

Pyright is working as designed here, so this isn't a bug.

You're redefining the class-scoped symbol age here, which isn't allowed by static type checkers. The two definitions for the symbol have conflicting types. That's not allowed by pyright (or other static type checkers).

No. age is not a attribute of my class, Its just in the generated init() function, since I used InitVar. Where do you see conflicting types?

try this:

from dataclasses import InitVar, dataclass, field


@dataclass
class Animal:
    name: str

    def alive(self) -> str:
        return f"{self.name} is alive"
@dataclass
class Eagle(Animal):
    age: InitVar[int]
    _age: int = field(init=False, repr=False)  # do not initialize, only register datafield
def main():
    e=Eagle(name= "Eddy",age= 1)
    print(e.age)
if __name__ == "__main__":
    main()

Pylance even notifies me:
Cannot access attribute "age" for class "Eagle"
  "age" is an init-only field

And at runtime I get this error:

Traceback (most recent call last):
  File "c:\Users\loren\.vscode\extensions\ms-python.python-2026.4.0-win32-x64\python_files\python_server.py", line 139, in exec_user_input
    retval = callable_(user_input, user_globals)
  File "<string>", line 1, in <module>
  File "<string>", line 16, in main
AttributeError: 'Eagle' object has no attribute 'age'