When you assign an object to a variable, Python copies the reference to the object.
In this case a and b refer to the same list.
If you want to copy the list (not just a reference to it), you can use the slice operator:
Making a slice of a creates a new list. in this case the slice contains all of the elements from the original list.
Another way to make a copy is the copy function from the copy module:
But is more idiomatic to use the slice operator.
from Thinking in Python
原文：http://www.cnblogs.com/ryansunyu/p/3841531.html