Python Prevent Re-assignment of a Class Method Outside The Class

January 27, 2022

I want some of my class methods to be not re-assignable outside (or both outside and inside) of the class. As functions are objects too in python, is it possible to accomplish what I want? Here is the method;

def x(self):
    """Return x, the horizontal distance between the shape and the leftmost of the screen, in pixels"""
    return self.__x

Now outside of the class, it is easy to mess with this method;

shape = Shape(10, 20)
shape.x = 30
print(shape.x)  # Prints 30
print(shape.x())    # TypeError: 'int' object is not callable

Is it possible to prevent this kind of assignment of class methods?

>Solution :

You could override the __setattr__ method to check to see if the key is in a forbidden list for example…

class Shape:
    def __init__(self, a, b):
        self.__x = a
        self.__y = b

    def x(self):
        return self.__x

    def __setattr__(self, key, value):
        if key in ('x', 'y'):
            raise TypeError(f"You can't change {key}")
        super(Shape, self).__setattr__(key, value)


shape = Shape(11, 20)
shape.z = 100  # OK
print(shape.z)
print(shape.x())
shape.x = 30  # raise an exception