Changing Variables in Class from Other Classes (Python)

Question

Say I have this code:

class hello_world():
    def define_hello(self):
        self.hello = "hello"
    def say_hello(self):
        print self.hello

class change_to_goodbye():
    def __init__(self):
        self.helloWorld = hello_world()
    def hello_to_goodbye(self):
        self.helloWorld.hello = "goodbye"

class other_class():
    def __init__(self):
        self.helloWorld = hello_world()
        self.changeToGoodbye = change_to_goodbye()

        self.helloWorld.define_hello()
        self.changeToGoodbye.hello_to_goodbye()
        self.helloWorld.say_hello()

oc = other_class()

Class hello_world has two methods, one that defines the variable hello and one that prints it. On the other hand, class change_to_goodbye tries to access the variable hello in class hello_world and changes it to goodbye. Class other_class should set the variable hello to "hello", change it to "goodbye", and print it on the screen.

I expected the output to be "goodbye" but I got "hello". Why isn't change_to_goodbye changing the variable of hello_world?

xgord · Accepted Answer · 2014-10-09 02:43:17Z

This is because your change_to_goodbye class stores its own hello_world object inside it. The hello_world object in changeToGoodbye is not the same as the hello_world object helloWorld. So in other_class when you do self.changeToGoodbye.hello_to_goodbye(), your helloWorld variable isn't being altered at all. What's being altered is changeToGoodbye's helloWorld object.

So: self.changeToGoodbye.helloWorld.say_hello() would return goodbye.

So to summarize:

class other_class():
    def __init__(self):
        self.helloWorld = hello_world() 
        self.changeToGoodbye = change_to_goodbye() #contains a different hello_world object

        self.helloWorld.define_hello()
        self.changeToGoodbye.hello_to_goodbye() #changes the hello_world object inside of changeToGoodbye
        self.helloWorld.say_hello() #the unaltered helloWorld object

if you wanted to change the output of helloWorld, you could change the change_to_goodbye class's constructor so that its hello_world object is the one you just created.

class change_to_goodbye():
    def __init__(self, hw):
            self.helloWorld = hw

    #other code

Is it possible for change_to_goodbye to directly change the variables in class hello_world though?
it does change the variables in hello_world, it's just changing a different instance of hello_world's variables. what you want to do is store the hello_world object you created inside the change_to_goodbye class. this can be done by altering the constructor in the manner I showed at the bottom of the answer

WKPlus · Accepted Answer · 2014-10-09 02:47:51Z

You are not changing class variable, you are changing instance variable.

By self.helloWorld = hello_world() you defines an hello_world instance let's called it A, and self.changeToGoodbye = change_to_goodbye() you defines an change_to_goodbye instance B which has an hello_world instance C.

Then self.helloWorld.define_hello() set A's variable as hello and self.changeToGoodbye.hello_to_goodbye() set C's variable as goodbye.

At last self.helloWorld.say_hello() will print A's variable, and you will get hello.

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