在C++里pass by value和pass by reference是兩個很容易區分的東西。但是在python中，讓我有點糊涂，其實Python是“pass by assignment”。
舉個例子：
>>> a=[1,2,3]
>>> b=a
>>> b
[1, 2, 3]
>>> a.append(4)
>>> b
[1, 2, 3, 4]
>>> a=[1]
>>> b
[1, 2, 3, 4]
可以看到，a和b一開始都是bind to [1,2,3]這一個list object。這個list object的變化（append 4）使得a和b都發生了變化。但是，當a bind to another object [1], b is still pointing to the original object, and the original object doesn't change.
This property properly determines when calling a function, how the parameter is passed. Personally I would understand that as "pass by object".
Reference
stack overflow and quora
