Python参数传递方式: call by sharing，means that each formal parameter of the function gets a copy of each reference in the arguments.
def f(a, b):
a += b
return a
>> a = (1, 2)
>> b = (3, 4)
>> f(a, b)
(1, 2, 3, 4)
>> a, b
>> (1, 2), (3, 4)
ref: Fluent Python
