Python - why use “self” in a class?

Question

How do these 2 classes differ?

class A():
    x=3

class B():
    def __init__(self):
        self.x=3

Is there any significant difference?

Answer 1

A.x is a class variable, and will be shared across all instances of A, unless specifically overridden within an instance. B.x is an instance variable, and each instance of B has its own version of it.

I hope the following Python example can clarify:

    >>> class Foo():
    ...     i = 3
    ...     def bar(self):
    ...             print 'Foo.i is', Foo.i
    ...             print 'self.i is', self.i
    ... 
    >>> f = Foo() # Create an instance of the Foo class
    >>> f.bar()
    Foo.i is 3
    self.i is 3
    >>> Foo.i = 5 # Change the global value of Foo.i over all instances
    >>> f.bar()
    Foo.i is 5
    self.i is 5
    >>> f.i = 3 # Override this instance's definition of i
    >>> f.bar()
    Foo.i is 5
    self.i is 3