z = 0

def calc0():
    z = 10
    print (locals())
    def twice():
        # UnboundLocalError: local variable 'z' referenced before assignment
        z *= 20
    twice()
    return z + 10

def calc1():
    z = 10
    def twice():
        global z
        z *= 20

    twice()
    return z + 10

def calc2():
    z = 10
    def twice():
        # UnboundLocalError: local variable 'z' referenced before assignment
        nonlocal z  ## this is the fix
        z *= 20

    twice()
    return z + 10

# print(calc0(10))