The operators we have seen so far are all special characters like + and *, but there are a few operators that are words. in is a boolean operator that takes two strings and returns True if the first appears as a substring in the second:
For example, the following function prints all the letters from word1 that also appear in word2:
With well-chosen variable names, Python sometimes reads like English. You could read this loop, ‘for (each) letter in (the first) word, if (the) letter (appears) in (the second) word, print (the) letter.
from Thinking in Python
The in operator,布布扣,bubuko.com
原文：http://www.cnblogs.com/ryansunyu/p/3815190.html