What is the difference.

[buran]

So you can change it, but if you do, you change the scope?

Well, that's not correct statement.

1. you can use global keyword. In this case you can change its value, you don't change the scope.

message="Message outside function"
print('value before function: {}'.format(message))
def foo():
    global message # THIS IS BAD STYLE
    message = "Message in function"
    print('value from function: {}'.format(message))
foo()
print('value after function: {}'.format(message))

Output:
value before function: Message outside function
value from function: Message in function
value after function: Message in function

2. as in your case, not using global, by assigning a value within the function, you effectively create different variable, with different (local) scope. So you don't change the value of the other variable named MESSAGE (one with the global scope).

message="Message outside function"
print('value before function: {}'.format(message))
def foo():
    message = "Message in function"
    print('value from function: {}'.format(message))
foo()
print('value after function: {}'.format(message))

Output:
value before function: Message outside function
value from function: Message in function
value after function: Message outside function