Global Variable

[Larz60+]

Therein lies the evil of global variables.
The answer to your question is yes and yes.

Consider the following,
The program will eventually fail precisely because you are allowed to change a global variable anywhere

MyGlobal = 17

def function_1(n):
    MyGlobal += n

def function_2():
    global MyGlobal
    print('MyGlobal is {}'.format(MyGlobal))

def function_3():
    global MyGlobal
    MyGlobal = str(MyGlobal)

def function_4():
    global MyGlobal
    for n in range(5):
        MyGlobal += n
        function_2()

def main():
    function_4()
    function_3()
    function_4()

if __name__ == '__main__':
    main()