字典的用法

Posted 2020-07-09 随心666

python 字典的一些操作

 

特点：无序，是唯一内置的映射类型。多用于实现哈希表或者关联数组。

key具有唯一性，可以使用固定长度的对象，不能使用列表、字典和包含可变长度类型的元组。访问形式：m[k]，k是key。如果找不到，报错：KeyError。

方法和操作如下：

 

项目	功能
len(m)	Key的长度
m[k]	字典m中，key为k的值
m[k]=x	字典m中，k的值为x
del m[k]	删除字典键
k in m	有没有k的key
m.clear()
m.copy()	浅复制
m.fromkeys(s [,value])
m.get(k [,v])
m.has_key(k)
m.items()
m.keys()
m.pop(k [,default])
m.popitem()
m.setdefault(k [, v])
m.update(b)
m.values()

 

＊构建字典

 

字典举例：

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

函数Dict可以从其他映射或者序列构建字典

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

>>> d = dict(items)

>>> d

{‘age‘: 42, ‘name‘: ‘Gumby‘}

>>> d[‘name‘]

‘Gumby‘

 

       也可以使用参数的方法：

>>> d = dict(name=‘Gumby‘, age=42)

>>> d

{‘age‘: 42, ‘name‘: ‘Gumby‘}

初始化空字典：

>>> x = {}

>>> x[42] = ‘Foobar‘

>>> x

{42: ‘Foobar‘}

 

＊格式化输出：

>>> phonebook

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

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

"Cecil‘s phone number is 3258."

 

 

>>> template = ‘‘‘

 

 

 

%(title)s

 

%(text)s

 

‘‘‘

>>> data = {‘title‘: ‘My Home Page‘, ‘text‘: ‘Welcome to my home page!‘}

>>> print template % data

My Home Page

Welcome to my home page!

另外string.Template 类也很适合这种应用。

＊字典方法：

-＊清除：clear

       下面例子展示clear和把字典置空字典的区别：

>>> x = {}

>>> y = x

>>> x[‘key‘] = ‘value‘

>>> y

{‘key‘: ‘value‘}

>>> x = {}

>>> y

{‘key‘: ‘value‘}

 

 

>>> x = {}

>>> y = x

>>> x[‘key‘] = ‘value‘

>>> y

{‘key‘: ‘value‘}

>>> x.clear()

>>> y

{}

 

 

-＊复制：copy

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

>>> y = x.copy()

>>> y[‘username‘] = ‘mlh‘

>>> y[‘machines‘].remove(‘bar‘)

>>> y

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

>>> x

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

 

     浅拷贝的元素指向原有元素，所以改变原有的可变元素，新的也会受影响，反之亦然。换种说法，对元素进行替换，不会对新老都产生影响，但是修改则会有影响。

 

>>> from copy import deepcopy

>>> d = {}

>>> d[‘names‘] = [‘Alfred‘, ‘Bertrand‘]

>>> c = d.copy()

>>> dc = deepcopy(d)

>>> d[‘names‘].append(‘Clive‘)

>>> c

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

>>> dc

{‘names‘: [‘Alfred‘, ‘Bertrand‘]}

 

-＊复制key：fromkeys

>>> {}.fromkeys([‘name‘, ‘age‘])

{‘age‘: None, ‘name‘: None}

       也可以用dict代替{}

>>> dict.fromkeys([‘name‘, ‘age‘])

{‘age‘: None, ‘name‘: None}

        设置其他默认值：

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

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

 

 

-＊获取：get 

>>> d = {}

>>> print d[‘name‘]

Traceback (most recent call last):

File "", line 1, in ?

KeyError: ‘name‘

>>> print d.get(‘name‘)

None

也可以用其他字符替代None

 

>>> d.get(‘name‘, ‘N/A‘)

‘N/A‘

 

-＊has_key：是否有key

和k in d是等效的，Python 3.0将取消这个，建议不要使用

>>> d = {}

>>> d.has_key(‘name‘)

False

>>> d[‘name‘] = ‘Eric‘

>>> d.has_key(‘name‘)

True

 

-＊列出值：items and iteritems

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

>>> d.items()

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

 

>>> it = d.iteritems()

>>> it

 

>>> list(it) # Convert the iterator to a list

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

       Iteritems生成迭代器，一般的情况下使用iteritems比iteritem更有效，尤其是循环的时候。

注：Python 3 items 返回的是迭代器

 

-＊列出关键字：keys and iterkeys

       后者是迭代器

根据key出栈：pop

>>> d = {‘x‘: 1, ‘y‘: 2}

>>> d.pop(‘x‘)

1

>>> d

{‘y‘: 2}

-＊出栈：popitem

>>> d

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

>>> d.popitem()

(‘url‘, ‘http://www.python.org‘)

>>> d

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

       出栈和列表和类似，但是没有append。

-＊setdefault：设置默认值。

>>> d = {}

>>> d.setdefault(‘name‘, ‘N/A‘)

‘N/A‘

>>> d

{‘name‘: ‘N/A‘}

>>> d[‘name‘] = ‘Gumby‘

>>> d.setdefault(‘name‘, ‘N/A‘)

‘Gumby‘

>>> d

{‘name‘: ‘Gumby‘}

       默认值的默认值为None。

>>> d = {}

>>> print d.setdefault(‘name‘)

None

>>> d

{‘name‘: None}

 

-＊更新：update

>>> 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‘}

 

 

-＊取值：values and itervalues

 

>>> d = {}

>>> d[1] = 1

>>> d[2] = 2

>>> d[3] = 3

>>> d[4] = 1

>>> d.values()

[1, 2, 3, 1]

 

 

＊实例：

存储个人的电话和地址的脚本：

 

# 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])

 

 

将上述例子改为getkey

 

# 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‘

    }

 

}

 

 

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)