def f():
    global city
    city = "Munich"

    def g():
        city = "Zurich"

    print("Before calling g: " + city)
    print("Calling g now:")
    g()
    print("After calling g: " + city)


city = "Stuttgart"
f()
print("'city' in main: " + city)

#output
#Before calling g: Munich
#Calling g now:
#After calling g: Munich
#'city' in main: Munich