Python global variable/scope confusion

I've started teaching myself python, and have noticed something strange to do with global variables and scope. When I run this:

x = 2 
y = 3 
z=17 
def add_nums():
    y = 6 
    return z+y

The result of 23 is printed... However, when I expand the return to be:

x = 2 
y = 3 
z=17 
def add_nums(): 
    y = 6 
    z = z + y 
    return z

I get the following error on line 6:

Local name referenced but not bound to a value.
A local name was used before it was created. You need to define the     
method or variable before you try to use it.

I'm confused as why I am getting an error here, as z is global an accessible.

When a variable is on the left side of an equal sign python will create a local variable. When the variable is on the right side of the equal sign python will try to look for a local variable and if he can't find one then it will use a global variable. In your example, z is on the right and left side of the equal sign, to avoid ambiguities python raises an error. You need to use the global syntax to avoid this:

x = 2 
y = 3 
z = 17 
def add_nums(): 
    global z
    y = 6 
    z = z + y 
    return z