The most common reason for this error is that you’re using multiple global declarations in the same function. Consider this example:

x = 0

def func(a, b, c):
    if a == b:
        global x
        x = 10
    elif b == c:
        global x
        x = 20

If you run this in a recent version of Python, the compiler will issue a SyntaxWarning pointing to the beginning of the func function.
Here’s the right way to write this:

x = 0

def func(a, b, c):
    global x # <- here
    if a == b:
        x = 10
    elif b == c:
        x = 20

參考：http://effbot.org/zone/syntaxwarning-name-assigned-to-before-global-declaration.htm