Python Variable Scope and Classes

In Python, if I define a variable:

my_var = (1,2,3)

and try to access it in __init__ function of a class:

class MyClass:
    def __init__(self):
        print my_var

I can access it and print my_var without stating (global my_var).

If I put my_var right after class MyClass however, I get scope error (no global variable found).

What is the reason for this? How should I do this? Where can I read about this to learn? I did read Python Class page but I did not encounter its explanation.

Thank you