Define a class with a 'bar' constant:

    >>> class Foo(object):
    ...     bar=0
    ... 
    >>>

Inspect that class level constant

    >>> Foo.bar
    0

Instantiate objects `a` and `b` from Foo. 

    >>> a = Foo()
    >>> b = Foo()
    >>> a.bar
    0
    >>> b.bar
    0

Turn `a.bar` into an instance variable by setting it manually.

    >>> a.bar = 1

Notice that the class-level variable hasn't changed

    >>> Foo.bar
    0
    >>> a.bar
    1

Now lets check out instance `b`..
    >>> b.bar
    0

Now change the class variable and look at `b.bar` variable and how it changed..

    >>> Foo.bar=2
    >>> b.bar
    2

Since we set an instance variable in `a`, it was not impacted...

    >>> a.bar
    1
    >>>