### 定义及应用
Python内置了字典：dict的支持，dict全称dictionary，在其他语言中也称为map，使用键-值（key-value）存储，具有极快的查找速度。
### 定义
~~~
stuff = {'name': 'Zed', 'age': 36, 'height': 6*12+2}#key:value pairs
~~~
### 词典的常用方法
print dic.keys() # 返回dic所有的键
print dic.values() # 返回dic所有的值
print dic.items() # 返回dic所有的元素（键值对）
dic.clear() # 清空dic，dict变为{}
del dic['tom'] # 删除 dic 的‘tom’元素
print(len(dic))
dict.get(key,default=None) 对字典dict 中的键key,返回它对应的值value，如果字典中不存在此键，则返回default 的值(注意，参数default 的默认值为None)
dict.has_key(key) 如果键(key)在字典中存在，返回True，否则返回False. 在Python2.2版本引入in 和not in 后，此方法几乎已废弃不用了，但仍提供一个 可工作的接口。
dict.keys() 返回一个包含字典中键的列表
dict.values() 返回一个包含字典中所有值的列表
### 实例
~~~
# create a mapping of state to abbreviation
states = {
'Oregon': 'OR',
'Florida': 'FL',
'California': 'CA',
'New York': 'NY',
'Michigan': 'MI'
}
# create a basic set of states and some cities in them
cities = {
'CA': 'San Francisco',
'MI': 'Detroit',
'FL': 'Jacksonville'
}
# add some more cities
cities['NY'] = 'New York'
cities['OR'] = 'Portland'
# print out some cities
print '-' * 10
print "NY State has: ", cities['NY']
print "OR State has: ", cities['OR']
# print some states
print '-' * 10
print "Michigan's abbreviation is: ", states['Michigan']
print "Florida's abbreviation is: ", states['Florida']
# do it by using the state then cities dict
print '-' * 10
print "Michigan has: ", cities[states['Michigan']]
print "Florida has: ", cities[states['Florida']]
# print every state abbreviation
print '-' * 10
for state, abbrev in states.items():
print "%s is abbreviated %s" % (state, abbrev)
# print every city in state
print '-' * 10
for abbrev, city in cities.items():
print "%s has the city %s" % (abbrev, city)
# now do both at the same time
print '-' * 10
for state, abbrev in states.items():
print "%s state is abbreviated %s and has city %s" % (
state, abbrev, cities[abbrev])
print '-' * 10
# safely get a abbreviation by state that might not be there
state = states.get('Texas', None)
if not state:
print "Sorry, no Texas."
# get a city with a default value
city = cities.get('TX', 'Does Not Exist')#if exist key=='TX',city=value,or city='Dose Not Exist'
print "The city for the state 'TX' is: %s" % city
~~~
output
~~~
----------
NY State has: New York
OR State has: Portland
----------
Michigan's abbreviation is: MI
Florida's abbreviation is: FL
----------
Michigan has: Detroit
Florida has: Jacksonville
----------
California is abbreviated CA
Michigan is abbreviated MI
New York is abbreviated NY
Florida is abbreviated FL
Oregon is abbreviated OR
----------
FL has the city Jacksonville
CA has the city San Francisco
MI has the city Detroit
OR has the city Portland
NY has the city New York
----------
California state is abbreviated CA and has city San Francisco
Michigan state is abbreviated MI and has city Detroit
New York state is abbreviated NY and has city New York
Florida state is abbreviated FL and has city Jacksonville
Oregon state is abbreviated OR and has city Portland
----------
Sorry, no Texas.
The city for the state 'TX' is: Does Not Exist
~~~
和list比较，dict有以下几个特点：
1. 查找和插入的速度极快，不会随着key的增加而增加；
1. 需要占用大量的内存，内存浪费多。
所以，dict是用空间来换取时间的一种方法。
dict可以用在需要高速查找的很多地方，在Python代码中几乎无处不在，正确使用dict非常重要，**需要牢记的第一条就是dict的key必须是**不可变对象**。这是因为dict根据key来计算value的存储位置，如果每次计算相同的key得出的结果不同，那dict内部就完全混乱了。
