While-loop scoping in Python 2.x

Question

I am new to Python, and here's a question that is really confusing me:

i = 1
def foo():
    while i < 5:
        print 'in while loop'
        i += 1
foo()

This code produces an error:

UnboundLocalError: local variable 'i' referenced before assignment

Why isn't the while loop seeing the global i ? As explained in the LEGB-rule, Python will search for the variable name sequentially until eventually searches in the global scope ?!

Why do I have to explicitly refer that i is global by adding global keyword:

i = 1
def foo():
    global i
    while i < 5:
       print 'in while loop'
       i += 1

On the other hand here foo prints the global value for i, '1' without adding global keyword.

i = 1
def foo():
    print 'i equals %d' % i