# parameters.defaults.mutable.intermediate.call.py
def func(a=[], b={}):
    print(a)
    print(b)
    print('#' * 12)
    a.append(len(a))  # this will affect a's default value
    b[len(a)] = len(a)  # and this will affect b's one

func()
func(a=[1, 2, 3], b={'B': 1})
func()