Python: How to Type Hint a Class Argument in a Static Method Python?

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Let's say I have a class Circle with the following definition:

class Circle:
    def __init__(self, r, _id):
        self.r = r
        self.id =  _id

    def area(self):
        return math.pi * (self.r ** 2)

I want to write a function that compares two circles and returns the id of the smallest one

def compare_circles(circle_1: Circle, circle_2: Circle) -> str:
    if circle_1.r < circle_2.r:
        return circle_1.id
    else:
        return circle_2.id

I would like to place this method as a static method on the class. (Is this a bad idea?)

class Circle:
    
    def __init__(self, r, _id):
        self.r = r
        self.id =  _id

    def area(self):
        return math.pi * (self.r ** 2)

    @staticmethod
    def compare_circles(circle_1: Circle, circle_2: Circle) -> str:
        if circle_1.r < circle_2.r:
            return circle_1.id
        else:
            return circle_2.id

However this does not work because the Circle object is not available to be type hinted. I have to remove the type hints in the method. Using a classmethod also does not work, as the cls argument is not available to the other method arguments.

type-hinting python-3.x python
1 Answers

A possible workaround in this case would be to monkey-patch the method after defining the class:

class Circle:
    def __init__(self, r, _id):
        self.r = r
        self.id =  _id

    def area(self):
        return math.pi * (self.r ** 2)


def compare_circles(circle_1: Circle, circle_2: Circle) -> str:
    if circle_1.r < circle_2.r:
        return circle_1.id
    else:
        return circle_2.id

Circle.compare_circles = staticmethod(compare_circles)
del compare_circles

The usual way would be to provide a string with the type name:

class Circle:
    def __init__(self, r, _id):
        self.r = r
        self.id =  _id

    def area(self):
        return math.pi * (self.r ** 2)


    @staticmethod
    def compare_circles(circle_1: 'Circle', circle_2: 'Circle') -> str:
        if circle_1.r < circle_2.r:
            return circle_1.id
        else:
            return circle_2.id

As an aside, you might also consider turning compare_circles into a method:

def compare(self, other: 'Circle') -> str:
    if self.r < other.r:
        return self.id
    else:
        return other.id
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