The most common way to traverse the elements of a list is with a for loop. The syntax is the same as for strings:
This works well if you only need to read the elements of the list. But if you want to write or update the elements, you need the indices. A common way to do that is to combine the functions range and len:
This loop traverses the list and updates each element. len returns the number of elements in the list. range returns a list of indices from 0 to n-1, where n is the length of the list. each time through the loop I gets the index of the next element. The assignment statement in the body uses I to read the old value of the element and to assign the new value.
A for loop over an empty list never executes the body:
Although a list can contain another list, the nested list still counts a single element. The length of this list is four:
from Thinking in Python
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原文：http://www.cnblogs.com/ryansunyu/p/3841087.html