How to get property return type annotation?

Question

I'm looking for a way to extract the return type annotation of a property from a class, e.g.:

class Test:
    @property
    def foo(self) -> int:
        return 1

print(return_type_extract(Test, 'foo')) # <class 'int'>

For standard methods this can be done like so:

import inspect
class Test:
    def foo(self) -> int:
        return 1

def return_type_extract(cls: type, method_name: str) -> type:
    method = getattr(cls, method_name)
    return inspect.signature(method).return_annotation

print(return_type_extract(Test, 'foo')) # <class 'int'>

This method however does not work for the @property decorator, as it raises an error inside inspect.signature. I've looked at an equivalent for properties in the inspect library, but so far no luck.

Answer 1

I hit the same issue and managed to find an answer, the key observation is here: https://docs.python.org/3/library/functions.html#property

fget is a function for getting an attribute value

This means signature return_annotation works, but we must apply it to properties' fget functions instead of directly to properties (as they encapsulate multiple functions)

import dataclasses
import inspect


@dataclasses.dataclass
class PropertyTest:
    a: int

    @property
    def b(self) -> float:
        return float(self.a)


def test_property_return_type_from_class():
    property_return_type = prop_return_type_from_class(
        cls=PropertyTest,
        prop_name="b",
    )
    assert property_return_type is float


def prop_return_type_from_class(
    *,
    prop_name: str,
    cls: type,
) -> type:
    prop = getattr(cls, prop_name)
    return_type = inspect.signature(prop.fget).return_annotation
    return return_type