python字典方法

dict()

1.以关键字参数建立字典

2.以其他映射作为参数建立字典

1.d = dict(name=‘Gumby‘, age=42)

　d

2.items = [(‘name‘, ‘Gumby‘), (‘age‘, 42)]

　d = dict(items)

　d

{‘age‘: 42, ‘name‘: ‘Gumby‘}

{‘age‘: 42, ‘name‘: ‘Gumby‘}

fromkeys()

1.给指定的键建立新的字典

2.用dict调用fromkeys

3.如果不想使用None作默认值，也可以自己提供默认值

1.{}.fromkeys([‘name‘, ‘age‘])

2.dict.fromkeys([‘name‘, ‘age‘])

3.dict.fromkeys([‘name‘,‘age‘], ‘(unknown)‘)

{‘age‘: None, ‘name‘: None}

{‘age‘: None, ‘name‘: None}

{‘age‘: ‘(unknown)‘, ‘name‘: ‘(unknown)‘}

clear()

1.清除字典中的所有项

2.无返回值（或返回值为None）

1.d = {‘name‘: ‘Gumby‘, ‘age‘: 42}

　value = d.clear()

　d

2. print value

{}

None

copy()

1.返回一个具有相同键值对的新字典

2.当在副本中替换值的时候，原始字典不受影响

3.当在副本中修改某个值（不是替换）时，原始字典会改变

1.x = {‘username‘: ‘admin‘, ‘machines‘: [‘foo‘, ‘bar‘, ‘baz‘]}

　y = x.copy()

　y

2.y[‘username‘] = ‘mhl‘

　y

　x

3.y[‘machines‘].remove(‘bar‘)

　y

　x

{‘username‘: ‘admin‘, ‘machines‘: [‘foo‘, ‘bar‘, ‘baz‘]}

{‘username‘: ‘mhl‘, ‘machines‘: [‘foo‘, ‘bar‘, ‘baz‘]}

{‘username‘: ‘admin‘, ‘machines‘: [‘foo‘, ‘bar‘, ‘baz‘]}

{‘username‘: ‘mhl‘, ‘machines‘: [‘foo‘, ‘baz‘]}

{‘username‘: ‘admin‘, ‘machines‘: [‘foo‘, ‘baz‘]}

deepcopy()

1.当在副本中修改某个值（不是替换）时，原始字典不受影响

from copy import deepcopy

d = {‘names‘: [‘Alfred‘, ‘Bertrand‘]}

deep = deepcopy(d)

deep[‘names‘].append(‘Clive‘)

deep

d

{‘names‘: [‘Alfred‘, ‘Bertrand‘, ‘Clive‘]}

{‘names‘: [‘Alfred‘, ‘Bertrand‘]}

get()

1.更宽松的访问字典项，访问一个不存在的键时，不会报错

2.自定义默认值

1.d = {}

　print d.get(‘name‘)

2.d.get(‘name‘, ‘N/A‘)

None

‘N/A‘

setdefault()

1.更宽松的访问字典项，访问一个不存在的键时，不会报错

2.自定义默认值

3.当键存在时，就返回相应的值

1.d = {}

　print d.setdefault(‘name‘)

2.d = {}

　d.setdefault(‘name‘, ‘N/A‘)

3.d = {‘name‘: ‘Gumby‘}

　d.setdefault(‘name‘, ‘N/A‘)

None

‘N/A‘

‘Gumby‘

has_key()

1.d.has_key(key)相当于k in d

d = {}

d.has_key(‘name‘)

d[‘name‘] = ‘Eric‘

d.has_key(‘name‘)

False

True

键的迭代：keys()和iterkeys()

1.keys()将字典中的键以列表形式返回

2.iterkeys()返回键的迭代器

1.d = {‘title‘: ‘Python Web Site‘, ‘url‘: ‘http://www.python.org‘, ‘spam‘: 0}

　d.keys()

2.ik = d.iterkeys()

　ik

　list(ik)

[‘url‘, ‘spam‘, ‘title‘]

<dictionary-keyiterator object at 0x0000000002C1FA98>

[‘url‘, ‘spam‘, ‘title‘]

值的迭代：values()和itervalues()

1.values()将字典中的值以列表形式返回

2.itervalues()返回值的迭代器

1.d = {‘title‘: ‘Python Web Site‘, ‘url‘: ‘http://www.python.org‘, ‘spam‘: 0}

　d.values()

2.iv = d.itervalues()

　iv

　list(iv)

[‘http://www.python.org‘, 0, ‘Python Web Site‘]

<dictionary-valueiterator object at 0x0000000002C1FAE8>

[‘http://www.python.org‘, 0, ‘Python Web Site‘]

键值对的迭代：items()和iteritems()

1.items()将所有字典项以列表形式返回

2.iteritems()返回所有字典项的迭代器

1.d = {‘title‘: ‘Python Web Site‘, ‘url‘: ‘http://www.python.org‘, ‘spam‘: 0}

　d.items()

2.it = d.iteritems()

　it

　list(it)

[(‘url‘, ‘http://www.python.org‘), (‘spam‘, 0), (‘title‘, ‘Python Web Site‘)]

<dictionary-itemiterator object at 0x0000000002C1FB38>

[(‘url‘, ‘http://www.python.org‘), (‘spam‘, 0), (‘title‘, ‘Python Web Site‘)]

pop()

1.获得给定键的值，然后将键值对从字典中移除

d = {‘x‘: 1, ‘y‘: 2|

d.pop(‘x‘)

d

1

{‘y‘: 2}

popitem()

1.移除随机的项

d = {‘url‘: ‘http://www.python.org‘, ‘spam‘: 0, ‘title‘: ‘Python Web Site‘}

d.popitem()

d

(‘url‘, ‘http://www.python.org‘)

{‘spam‘: 0, ‘title‘: ‘Python Web Site‘}

update()

1.利用一个字典项更新另外一个字典

d = {‘title‘: ‘Python Web Site‘, ‘url‘: ‘http://www.python.org‘, ‘changed‘: ‘Mar 14 22:09:15 MET 2008‘}

x = {‘title‘: ‘Python Language Website‘}

d.update(x)

d

{‘url‘: ‘http://www.python.org‘, ‘changed‘: ‘Mar 14 22:09:15 MET 2008‘, ‘title‘: ‘Python Language Website‘}