behavior of classmethods and static variables in pythone

Question

Suppose I have the following classes

class Parent: 
    message = "Hello World"
    @classmethod
    def print_message(cls):
        print(cls.message)
    @classmethod 
    def change_message(cls, new_message):
            cls.message =  new_message

class Child_one(Parent):
    @classmethod 
    def print_messsage(cls):
        print (cls.message)
    @classmethod 
    def change_message(cls, new_message):
        cls.message = new_message

class Child_two(Parent):
    @classmethod
    def print_message(cls):
        print(cls.message)

I suppose my question is to what does the cls of child classes refer to, because If I change the static variable message in parent class it rightly changes values of the of message in both children

Parent.change_message("This is new message")
Child_one.print_message() # This is new message 
Child_two.print_message() # This is new message

this is my expected behavior, both of the cls.message refer to the message of parent class. However doing this

Child_one.change_message("I am child one")

results in behavior such as

Parent.print_message() # This is new message 
Child_one.print_message() # I am child one 
Child_two.print_message() # This is new message

So the cls.message variable of Child_one no longer points to the message of the parent class.

In other langues such as c++ or c# static changing a static variable of a parent class or a child class results in change in all of the parent and derived classes. Is there a way to duplicate this behavior in python and if not why not?

Frank-Rene Schäfer · Accepted Answer · 2019-11-16 07:55:37Z

1

Upon Write:

As soon as you assign a something to a member of a class or object while this member does not exist by name, it is created.

The fact, that a member's name is present in the base class does not change anything. It is created in the class itself.

Upon Read:

The name resolution goes then: First try to find a member in the class itself, then in its base classes.

answered Nov 16, 2019 at 7:55

Frank-Rene Schäfer

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1 Comment

Vahagn Tumanyan Over a year ago

I understand that, however I don't understand why does python create a new member for Child_one when I set it. It doesn't exist for the Child_one, but since it's a derived class shouldn't it check if it's parent class has a member called message and only then decide to create it? Doesn't ignoring parent classes upon write violate the is a relationship

Jacob A. Bridges · Accepted Answer · 2019-11-18 20:29:48Z

0

Frank-Rene's answer is absolutely correct, and you can see this behavior by observing the memory addresses of the class variable:

print(Parent.message, Child_one.message) #identical memory addresses
Child_one.change_message("something")
print(Parent.message, Child_one.message) #now they are different

This is because each class in Python keeps its attributes in a dunder dictionary (__dict__), and if they are not overriden they will reference the parent until modified.

For your use case, you might be interested in Python's borg design pattern: https://github.com/faif/python-patterns/blob/master/patterns/creational/borg.py

edited Nov 18, 2019 at 20:29

answered Nov 16, 2019 at 17:44

Jacob A. Bridges

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