字典类型

一、映射：通过任意键查找集合中值信息的过程，python中通过字典实现映射。字典是键值对的集合。

passwd={"China":"BigCountry","Korean":"SmallCountry","France":"MediumCountry"}

print passwd

运行结果：
{"China":"BigCountry","Korean":"SmallCountry","France":"MediumCountry"}
二、字典的基本操作：
（1）为字典增加一项
dictionaryName[key]=value
（2）删除字典中的一项
del dictionaryName[key]
（3）字典的遍历

for key in students:
    print(key+":"+str(students[key]))

（4）遍历字典的键key

for key in dictionaryName.keys():
    print(key)

（5）遍历字典的值value

for value in dictionaryName.values():
    print(value)

（6）遍历字典的项

passwd={"China":"BigCountry","Korean":"SmallCountry","France":"MediumCountry"}
for item in passwd.items():
    print(item)

运行结果：

('China', 'BigCountry')
('Korean', 'SmallCountry')
('France', 'MediumCountry')

（7）遍历字典的key-value

passwd={"China":"BigCountry","Korean":"SmallCountry","France":"MediumCountry"}
for item,value in passwd.items():
    print(item,value)

运行结果：

China BigCountry
Korean SmallCountry
France MediumCountry

（8）判断一个键是否在字典中in或not in

>>>passwd={"China":"BigCountry","Korean":"SmallCountry","France":"MediumCountry"}

>>>"China" in passwd
True
>>>"America" in passwd
False

（9）clear()删除字典中的所有项目
get(key)返回字典中key对应的值
pop(key)删除并返回字典中key对应的值
update(字典)将字典中的键值添加到字典中

>>>passwd={"China":"BigCountry","Korean":"SmallCountry","France":"MediumCountry"}
>>>passwd.get("China")
"BigCountry"
>>>passwd.pop(Korean)
"SmallCountry"
>>>passwd
{"China":"BigCountry","France":"MediumCountry"}
>>>passwd.clear()
>>>passwd
{}