#This causes an error!  Because function f accesses a variable called
# y, but there is no local variable y in f, and there is no global variable
# y.  The only y is a local variable in the function called main.
def f(x):
    print(y)
    return x * 2

def main():
   y = 3
   z = f(4)
   print(z)

main()