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字典中的键使用时必须满足一下两个条件：
1、每个键只能对应一个项，也就是说，一键对应多个值时不允许的（列表、元组和其他字典的容器对象除外）。当有键发生冲突时（即字典键重复赋值），取最后的赋值。
>>> myuniversity_dict = {‘name‘:‘yuanyuan‘, ‘age‘:18, ‘age‘:19, ‘age‘:20, ‘schoolname‘:Chengdu, ‘schoolname‘:Xinxiang}
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name ‘Chengdu‘ is not defined
>>> myuniversity_dict = {‘name‘:‘yuanyuan‘, ‘age‘:18, ‘age‘:19, ‘age‘:20, ‘schoolname‘:‘Chengdu‘, ‘schoolname‘:‘Xinxiang‘}
>>> myuniversity_dict
{‘age‘: 20, ‘name‘: ‘yuanyuan‘, ‘schoolname‘: ‘Xinxiang‘}
>>>
2、键必须是可哈希的，像列表和字典这样的可变类型，由于他们是不可哈希的，所以不能作为字典的键。
为什么呢？—— 解释器调用哈希函数，根据字典中键的值来计算存储你的数据的位置。如果键是可变对象，可以对键本身进行修改，那么当键发生变化时，哈希函数会映射到不同的地址来存储数据，这样哈希函数就不可能可靠地存储或获取相关的数据; 选择可哈希键的原因就是他们的值不能被改变。摘抄python 核心编程（第二版）的一个实例如下：
#!/usr/bin/env python db = {} def newuser(): prompt = ‘login desired: ‘ while True: name = raw_input(prompt) if db.has_key(name): prompt = ‘name taken, try another\n‘ continue else: break pwd = raw_input(‘passwd: ‘) db[name] = pwd def olduser(): name = raw_input(‘login: ‘) pwd = raw_input(‘passwd: ‘) passwd = db.get(name) if passwd == pwd: print ‘welcome back‘, name else: print ‘login incorrect‘ def showmenu(): prompt = """ (N)ew User Login (E)xisting User Login (Q)uit Enter choice:""" done = False while not done: chosen = False while not chosen: try: choice = raw_input(prompt).strip()[0].lower() except: choice = ‘q‘ print ‘\nYou picked: [%s]‘ % choice if choice not in ‘neq‘: print ‘invalid option, try again‘ else: chosen = True if choice == ‘q‘:done = True if choice == ‘n‘:newuser() if choice == ‘e‘:olduser() if __name__ == ‘__main__‘: showmenu()
运行结果：
[root@localhost src]# python usrpw.py (N)ew User Login (E)xisting User Login (Q)uit Enter choice:n You picked: [n] login desired: root passwd: 1 (N)ew User Login (E)xisting User Login (Q)uit Enter choice:n You picked: [n] login desired: root name taken, try another
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原文地址：http://www.cnblogs.com/noxy/p/5721889.html
