How to combine property and function attributes in python?

Question

Let's say there is a class SomeClass with function func which returns int. How can we implement a similar interface?

class SomeClass:
    @overload
    @property
    def f(self) -> int:
        return 1

    @overload
    def f(self, x: int) -> int:
        return x


if __name__ == "__main__":
    obj = SomeClass()
    print(obj.f)  # print 1
    print(obj.f(5))  # print 5

Preferably a solution without external modules, but if this is problematic, it is acceptable.

P.S. overload decorator doesn't exist, this is just an example.

Edit:

Function overloading is given only as an example implementation, but it is not necessary. The main goal is to implement this interface type:

obj = SomeClass()
obj.f  # returns 1 (without brackets, works like property)
obj.f(5)  # returns 5 (with brackets, works like a normal function)

There is such a thing as @overload in Python, but you can't use it to have obj.f be both an int and a callable. — Samwise
– Samwise, Commented Dec 7, 2021 at 0:35
@Samwise right, it is merely for type hinting, it doesn't actually allow for ad hoc polymorphism — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Dec 7, 2021 at 0:38
You can also combine it with *args, **kwargs to have a method that behaves differently (and has different type signatures) depending on the type of its arguments. But that still doesn't help OP with what they're trying to do. That requires significantly more shenanigans. — Samwise
– Samwise, Commented Dec 7, 2021 at 0:39
dang, it got closed right before I could post the answer to how you actually do this (it involves magic methods on the object returned by the property) :P — Samwise
– Samwise, Commented Dec 7, 2021 at 0:41

Samwise · Accepted Answer · 2021-12-07 00:49:49Z

2

The core of your problem here is that you want obj.f to be simultaneously an int and a function:

    print(obj.f)     # print 1
    print(obj.f(5))  # print 5

This is technically possible by creating a callable int with the desired behavior, and having your property return that:

class SomeClass:
    class TrickyInt(int):
        def __new__(cls, val):
            return super().__new__(cls, val)

        def __call__(self, val):
            return self.__class__(val)

    @property
    def f(self):
        return self.TrickyInt(1)


if __name__ == "__main__":
    obj = SomeClass()
    print(obj.f)     # prints 1
    print(obj.f(5))  # prints 5

This is a lot of trouble to go to in order to implement a very unintuitive interface, though. I would recommend having this be a regular old method:

class SomeClass:
    def f(self, val=1):
        return val


if __name__ == "__main__":
    obj = SomeClass()
    print(obj.f())   # prints 1
    print(obj.f(5))  # prints 5

answered Dec 7, 2021 at 0:49

Samwise

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2 Comments

juanpa.arrivillaga Over a year ago

This should almost certainly never be used

Samwise Over a year ago

beware the magic!

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