from collections import deque

def person_is_mangoSeller(name):
    return name[-1] == 'm'

graph = {}
graph["you"] = ["Alice"]
graph["Alice"] = ["Cindy","you"]
graph["Cindy"] = ["Tom"]

def search(name):
    search_deque = deque()
    search_deque += graph[name]#graph变为graph[name]
    searched = []
    while search_deque:
        person = search_deque.popleft()#popleft() takes no arguments
        if person not in searched:
            if person_is_mangoSeller(person):
                print name + " " + "is a mango seller"
                return True
            else:
                search_deque += graph[person]
                searched.append(person)
    return False

search("you")

结果显示“you is a mango seller”。本来一直不懂：我不是已经创造了一个数组放检查过的人了吗？？怎么还会出现you呢？
后来经过不断的调试才发现：search("you")时，检查的其实是Alice，跟“you”是没有关系的。
但是还是不对啊！问题不在这里。
又调试一遍，原来，是19行中name的问题，因为在整个过程中，我传入的name是永远不变的，都是“you”，变的是被推出的person。所以，正确的代码应该是：

#查找芒果商
from collections import deque

def person_is_mangoSeller(name):
    return name[-1] == 'm'

graph = {}
graph["you"] = ["Alice"]
graph["Alice"] = ["Cindy","you"]
graph["Cindy"] = ["Tom"]

def search(name):
    search_deque = deque()
    search_deque += graph[name]#graph变为graph[name]
    searched = []
    while search_deque:
        person = search_deque.popleft()#popleft() takes no arguments
        if person not in searched:
            if person_is_mangoSeller(person):
                print person + " " + "is a mango seller"
                return True
            else:
                search_deque += graph[person]
                searched.append(person)
    return False

search("you")

好吧，虽然不是什么含金量高的错误，但解决了还是很有意义。