There really is no reason to create a new object in other to utilize the properties and methods of another object.

You only want to create an instance of Class_A in File_B to use it's properties and methods.

For example:

import Class_A

#instance of Class_A
classA = Class_A('params')

#property1 is a property in classA
classA.property1

#doSomething is a method in classA
classA.doSomething()

Read more about OOP here http://www.python-course.eu/object_oriented_programming.php

If I understand the problem correctly, then you just need to pass your instance of t1 to an instance of t2.

class t2:
    def getVar(self, t1):
        print t1.returnVar(t1)

t1_instance = t1()
t2_instance = t2()

t2.getVar(t1_instance)

Yes, that's exactly what you should expect. When you define a variable on the class, then those attributes are attached to the class, not the instance. You might not always notice that when you assign to an attribute on an instance, the instance picks up the new value, masking the class attribute. Methods that modify in place, like list.append won't give the instance a new attribute, since they just modify existing object, which happens to be an attribute of the class.

Any time every instance of a class should have its own, unique value for an attribute, you should usually set that in the __init__ method, to be sure that it's different for every instance.

only when a class has an attribute that has a sensible default value, and that value is of an immutable type (that cannot be modified in place), such as int or str, should you set attributes on the class.

Here is a very good post, and summary it as below.

Copyclass Bar(object):
    ## No need for dot syntax
    class_var = 1

    def __init__(self, i_var):
        self.i_var = i_var

## Need dot syntax as we've left scope of class namespace
Bar.class_var
## 1
foo = Bar(2)

## Finds i_var in foo's instance namespace
foo.i_var
## 2

## Doesn't find class_var in instance namespace…
## So look's in class namespace (Bar.__dict__)
foo.class_var
## 1

And in visual form

Class attribute assignment

• If a class attribute is set by accessing the class, it will override the value for all instances

  Copy  foo = Bar(2)
    foo.class_var
    ## 1
    Bar.class_var = 2
    foo.class_var
    ## 2
  

• If a class variable is set by accessing an instance, it will override the value only for that instance. This essentially overrides the class variable and turns it into an instance variable available, intuitively, only for that instance.

  Copy  foo = Bar(2)
    foo.class_var
    ## 1
    foo.class_var = 2
    foo.class_var
    ## 2
    Bar.class_var
    ## 1
  

When would you use class attribute?

• Storing constants. As class attributes can be accessed as attributes of the class itself, it’s often nice to use them for storing Class-wide, Class-specific constants

  Copy  class Circle(object):
         pi = 3.14159
  
         def __init__(self, radius):
              self.radius = radius   
        def area(self):
             return Circle.pi * self.radius * self.radius
  
    Circle.pi
    ## 3.14159
    c = Circle(10)
    c.pi
    ## 3.14159
    c.area()
    ## 314.159
  

• Defining default values. As a trivial example, we might create a bounded list (i.e., a list that can only hold a certain number of elements or fewer) and choose to have a default cap of 10 items

  Copy  class MyClass(object):
        limit = 10
  
        def __init__(self):
            self.data = []
        def item(self, i):
            return self.data[i]
  
        def add(self, e):
            if len(self.data) >= self.limit:
                raise Exception("Too many elements")
            self.data.append(e)
  
     MyClass.limit
     ## 10
  

First of all, you should rewrite like this:

class User(object):
  def __init__(self, username, password):
    self.username = username
    self.password = password

This way, username and password are instance variables instead of class variables (in your example, they are class variables -- all instance variables need to be defined in __init__ as properties of self). Then, you can initialize a User with whatever username and password you want, including None if you truly want them to have no value.