function - Incrementing integer variable of global scope in Python

I am trying to change global value x from within another functions scope as the following code shows,

x = 1
def add_one(x):

    x += 1

then I execute the sequence of statements on Python's interactive terminal as follows.

>>> x
1
>>> x += 1
>>> x
2

>>> add_one(x)
>>> x
2

Why is x still 2 and not 3?