In Python what is a global statement?

Question

What is a global statement? And how is it used? I have read Python\'s official definition;
however, it doesn\'t make a lot of sense to me.

Answer 1

mgilson did a good job but I'd like to add some more.

list1 = [1]
list2 = [1]

def main():
    list1.append(3)
    #list1 = [9]
    list2 = [222]

    print list1, list2


print "before main():", list1, list2
>>> [1] [1]
main()
>>> [1,3] [222]
print list1, list2    
>>> [1, 3] [1]

Inside a function, Python assumes every variable as local variable unless you declare it as global, or you are accessing a global variable.

list1.append(2) 

was possible because you are accessing the 'list1' and lists are mutable.

list2 = [222]

was possible because you are initializing a local variable.

However if you uncomment #list1 = [9], you will get

UnboundLocalError: local variable 'list1' referenced before assignment

It means you are trying to initialize a new local variable 'list1' but it was already referenced before, and you are out of the scope to reassign it.

To enter the scope, declare 'list1' as global.

I strongly recommend you to read this even though there is a typo in the end.