global - To declare a global variable

Variables defined outside of a function or class are global but read-only, also known as "constants."  The global keyword tells Python that the variable is read-writeable - it is not being treated as a constant. 

Example

Python
q = 10 # A global variable
def addToQ():
    global q # We want to change it
    q = q + 5

addToQ() 
print(q)

Output

15

Notes: Use with caution!  Global variables usually should never be changed because this can cause side-effects when other code expects the global variable to behave like a constant.