在C++里pass by value和pass by reference是兩個(gè)很容易區(qū)分的東西。但是在python中，讓我有點(diǎn)糊涂，其實(shí)Python是“pass by assignment”。

舉個(gè)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(gè)list object。這個(gè)list object的變化（append 4）使得a和b都發(fā)生了變化。但是，當(dāng)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