第4章 字典：当索引不好用时

4.1 创建和使用字典

>>> phonebook={'Alice':'2341','Beth':'9102','Cecil':'3258'}

>>> phonebook

{'Beth': '9102', 'Alice': '2341', 'Cecil': '3258'}

注意：字典中的键是唯一的（其他类型的映射也是如此），但值并不唯一。

 

dict函数：通过其他映射（比如其他字典）或者（键，值）这样的序列对建立字典。

>>> items=[('liyuan',34),('liming',35)]

>>> d=dict(items)

>>> d

{'liming': 35, 'liyuan': 34}

>>> items2=(('liyuan',34),('liming',35))

>>> e=dict(items2)

>>> e

{'liming': 35, 'liyuan': 34}

dict函数也可以通过关键字参数来创建字典：

>>> d=dict(name='Gumby',age=42)

>>> d

{'age': 42, 'name': 'Gumby'}

 

成员资格： 表达式k  in   d （d为字典），查找的是键，而不是值。

 

字典示例：

# A simple database

 

# A dictionary with person names as keys. Each person is represented as

# another dictionary with the keys 'phone' and 'addr' referring to their phone

# number and address, respectively.

 

people = {

 

    'Alice': {

        'phone': '2341',

        'addr': 'Foo drive 23'

    },

 

    'Beth': {

        'phone': '9102',

        'addr': 'Bar street 42'

    },

 

    'Cecil': {

        'phone': '3158',

        'addr': 'Baz avenue 90'

    }

 

}

 

# Descriptive labels for the phone number and address. These will be used

# when printing the output.

labels = {

    'phone': 'phone number',

    'addr': 'address'

}

 

name = raw_input('Name: ')

 

# Are we looking for a phone number or an address?

request = raw_input('Phone number (p) or address (a)? ')

 

# Use the correct key:

if request == 'p': key = 'phone'

if request == 'a': key = 'addr'

 

# Only try to print information if the name is a valid key in

# our dictionary:

if name in people: print "%s's %s is %s." % \

(name, labels[key], people[name][key])

4.2 字典的格式化字符串

>>> phonebook={'Beh':'9102','Alice':'2341','Ceil':'3258'}

>>> phonebook

{'Beh': '9102', 'Alice': '2341', 'Ceil': '3258'}

>>> "Cecil's phone number is %(Ceil)s." % phonebook

"Cecil's phone number is 3258."

4.3 字典的方法

clear方法：

也可以直接在所有字典的类型dict上面调用方法

>>> dict.fromkeys(['name','age'])

{'age': None, 'name': None}

如果不想使用None作为默认值，也可以自己提供默认值。

>>> dict.fromkeys(['name','age'],'(unknow)')

{'age': '(unknow)', 'name': '(unknow)'}

>>> dict.fromkeys(['name','age'],'unknow')

{'age': 'unknow', 'name': 'unknow'}

 

get方法：一个更宽松的访问字典项的方法。

# A simple database using get()

 

# Insert database (people) from Listing 4-1 here.

 

labels = {

    'phone': 'phone number',

    'addr': 'address'

}

 

name = raw_input('Name: ')

 

# Are we looking for a phone number or an address?

request = raw_input('Phone number (p) or address (a)? ')

 

# Use the correct key:

key = request # In case the request is neither 'p' nor 'a'

if request == 'p': key = 'phone'

if request == 'a': key = 'addr'

 

# Use get to provide default values:

person = people.get(name, {})

label = labels.get(key, key)

result = person.get(key, 'not available')

 

print "%s's %s is %s." % (name, label, result)

7 keys和iterkeys

   keys方法将字典中的键以列表形式返回，而iterkeys则返回针对键的迭代器。

>>> d={'x':1,'y':2}

8 pop方法

>>> d.pop('x')

1

>>> d

{'y': 2}