Changing class attributes by reference

Question

I'm relatively new to Python and have problems with immutable variables.

I'm trying to change the value of a class attribute (e.g. car.color). The difficulty is, that I can not use the namespace of car for doing this.

Up to now I did not find a satisvying answer to my questions. In the code below I tried to summarize the possible solutions (workarrounds) I found and their disadvantages:

class Car:

    def __init__(self):
        self.color = "green"
        self.color_list = ["green"]
        self.color_attrib = "green"
        self.name = "VW Golf"
        """
        and many more attributes...
        """

    def makesetter(self, attribute):
        def set_value(value):
            attribute=value
        return set_value

    def set_color(self, value):
        "in this function I directly have access to car.color and can change its value: "
        self.color = value

    def set_attrib(self, attribute_string, value):
        setattr(self,attribute_string,value)

def change_attribute(attribute, value):
    "In this function I can not access car.color directly"
    attribute=value

def change_attribute_list(attribute, value):
    "In this function I can not access car.color directly"
    attribute[0] = value



if __name__ == "__main__":

    car1 = Car()

    change_attribute(car1.color, "red")
    print(car1.color)  # Color does not change because car1.color is immutable

    g = car1.makesetter(car1.color)
    g("red")
    print(car1.color)   # Color does not change because car1.color is immutable

    change_attribute_list(car1.color_list, "red")
    print(car1.color_list)  # Color changes but seems like a workarround
    # Disadvantage: in the namespace of car1, the user has to use a list to access a string value "car1.color_list[0]"

    car1.set_color("red")
    print(car1.color)  # Color changes but seems like a workarround
    # Disadvantage: Car needs a setter function for each attribute

    car1.set_attrib("color_attrib","red")
    print(car1.color_attrib)  # Color changes but seems like a workarround
    # Disadvantage: Attribute has to be passed as string & no auto completion while coding

Actually the function setattr() is internally exactly doing what I want. But it works with a string argument. I tried to look into this function but it seems to be written in C++.

So do I have to use C++ to solve this problem without a workarround? Or is there a Pythionic way of doing this?

Answer 1

The problem is you are trying to redefine the value of an instance from outside of the class. Since in __init__ you are defining your variables with self, they are only available for that instance. This is the point of a class - it's what makes them extensible and reusable.

Ideally, you would make a method within the class that would update those attributes, however, if you really need to update the class from an external function, you will have to define it as a class level variable. For instance:

class Car:

    def __init__(self):
        Car.color = "green"

can now be updated using:

def change_attribute(attribute, value):
    "In this function I can not access car.color directly"
    Car.color=value

outside of the class because you have not assigned it to one specific instance. Doing this presents a problem, however. Since we don't have a separate instance variable, if we try to re-instantiate the class, we are stuck with what was previously changed, i.e. if name == "main":

car1 = Car()
car2 = Car()
change_attribute(car1.color, "red")
print(car1.color)  # Prints red
print(car2.color)  # Prints red

change_attribute(car2.color, "blue")
print(car1.color)  # Prints blue
print(car2.color)  # Prints blue

This is why classes themselves should be self contained and are meant to be immutable - the instance itself should be changed.