How would I access variables from one class to another?

I am writing a program that is utilizing multiple classes. I have one class that is dedicated to determining values for a set of variables. I would then like to be able to access the values of those variables with other classes. My code looks as follows:

class ClassA(object):
    def __init__(self):
        self.var1 = 1
        self.var2 = 2

    def methodA(self):
        self.var1 = self.var1 + self.var2
        return self.var1


class ClassB(ClassA):
    def __init__(self):
        self.var1 = ?
        self.var2 = ?

object1 = ClassA()
sum = object1.methodA()
print sum

I use classA to initialize 2 variables (var1 and var2). I then use methodA to add them, saving the result as var1 (I think this will make var1 = 3 and var2 = 2). What I want to know is how would I have ClassB then be able to get the values for var1 and var2 from ClassA?

Solution 1:

var1 and var2 are instance variables. That means that you have to send the instance of ClassA to ClassB in order for ClassB to access it, i.e:

class ClassA(object):
    def __init__(self):
        self.var1 = 1
        self.var2 = 2

    def methodA(self):
        self.var1 = self.var1 + self.var2
        return self.var1



class ClassB(ClassA):
    def __init__(self, class_a):
        self.var1 = class_a.var1
        self.var2 = class_a.var2

object1 = ClassA()
sum = object1.methodA()
object2 = ClassB(object1)
print sum

On the other hand - if you were to use class variables, you could access var1 and var2 without sending object1 as a parameter to ClassB.

class ClassA(object):
    var1 = 0
    var2 = 0
    def __init__(self):
        ClassA.var1 = 1
        ClassA.var2 = 2

    def methodA(self):
        ClassA.var1 = ClassA.var1 + ClassA.var2
        return ClassA.var1



class ClassB(ClassA):
    def __init__(self):
        print ClassA.var1
        print ClassA.var2

object1 = ClassA()
sum = object1.methodA()
object2 = ClassB()
print sum

Note, however, that class variables are shared among all instances of its class.

Solution 2:

Can you explain why you want to do this?

You're playing around with instance variables/attributes which won't migrate from one class to another (they're bound not even to ClassA, but to a particular instance of ClassA that you created when you wrote ClassA()). If you want to have changes in one class show up in another, you can use class variables:

class ClassA(object):
   var1 = 1
   var2 = 2
   @classmethod
   def method(cls):
       cls.var1 = cls.var1 + cls.var2
       return cls.var1

In this scenario, ClassB will pick up the values on ClassA from inheritance. You can then access the class variables via ClassA.var1, ClassB.var1 or even from an instance ClassA().var1 (provided that you haven't added an instance method var1 which will be resolved before the class variable in attribute lookup.

I'd have to know a little bit more about your particular use case before I know if this is a course of action that I would actually recommend though...

Solution 3:

var1 and var2 is an Instance variables of ClassA. Create an Instance of ClassB and when calling the methodA it will check the methodA in Child class (ClassB) first, If methodA is not present in ClassB you need to invoke the ClassA by using the super() method which will get you all the methods implemented in ClassA. Now, you can access all the methods and attributes of ClassB.

class ClassA(object):
    def __init__(self):
        self.var1 = 1
        self.var2 = 2

    def methodA(self):
        self.var1 = self.var1 + self.var2
        return self.var1


class ClassB(ClassA):
    def __init__(self):
        super().__init__()
        print("var1",self.var1)
        print("var2",self.var2)


object1 = ClassB()
sum = object1.methodA()
print(sum)

Solution 4:

class ClassA(object):
    def __init__(self):
        self.var1 = 1
        self.var2 = 2
    def method(self):
        self.var1 = self.var1 + self.var2
        return self.var1

class ClassB(ClassA):
    def __init__(self):
        ClassA.__init__(self)

object1 = ClassA() 
sum = object1.method()  
object2 = ClassB() 
print sum