python学习笔记（十四）之字典

字典：是python中唯一的映射类型，字典中每一项都是由键-值对组成的项。字典中没有索引，只有键和值。键的类型可以是整型，变量或字符串。

创建和访问字典：

 >>> dict1 = {'Lining':'Anything is possible', 'Nike':'Just do is', 'Adidas':'Impossible is nothing'}
 >>> for i in dict1:
 ...     print(i)
 ...
 Lining
 Adidas
 Nike
 >>> for i in dict1:
 ...     print(dict1[i])
 ...
 Anything is possible
 Impossible is nothing
 Just do is
 >>> dict2 = {1:'one',2:'two',3:'three'}
 >>> dict2[3]
 'three'
 >>> dict2[1]
 'one'
 >>> dict2[0]
 Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
 KeyError: 0
 >>> dict3 = dict((('a',97),('b',98),('c',99)))
 >>> dict3
 {'c': 99, 'b': 98, 'a': 97}
 >>> dict4 = dict(Jobs = 'stay hungry, stay foolish', Agan = 'run,run,run')
 >>> dict4
 {'Agan': 'run,run,run', 'Jobs': 'stay hungry, stay foolish'}

通过键可以访问和修改对应的值，若对一个不存在的键进行赋值，会创建一个新项。

 >>> dict1
 {'Lining': 'Anything is possible', 'Adidas': 'Impossible is nothing', 'Nike': 'Just do is'}
 >>> dict1['Lining'] = 'nothing is nothing'
 >>> dict1
 {'Lining': 'nothing is nothing', 'Adidas': 'Impossible is nothing', 'Nike': 'Just do is'}
 >>> dict1['nothing'] = 'nothing is nothing'
 >>> dict1
 {'Lining': 'nothing is nothing', 'Adidas': 'Impossible is nothing', 'Nike': 'Just do is', 'nothing': 'nothing is nothing'}

fromkeys()方法

dict.fromkeys(s[, v])  用s中的值作为键，v作为对应的值创建一个新的字典。

 >>> dict1 = {}
 >>> dict1
 {}
 >>> dict1.fromkeys([1,2,3],(1,2,3))
 {1: (1, 2, 3), 2: (1, 2, 3), 3: (1, 2, 3)}
 >>> dict1.fromkeys([1,2,3],'number')
 {1: 'number', 2: 'number', 3: 'number'}

keys() 返回字典键的引用

values() 返回字典的值的引用

items() 返回对应项的引用

 >>> dict1 = dict1.fromkeys(range(32), '赞')
 >>> dict1
 {0: '赞', 1: '赞', 2: '赞', 3: '赞', 4: '赞', 5: '赞', 6: '赞', 7: '赞', 8: '赞', 9: '赞', 10: '赞', 11: '赞', 12: '赞', 13: '赞', 14: '赞', 15: '赞', 16: '赞', 17: '赞', 18: '赞', 19: '赞', 20: '赞', 21: '赞', 22: '赞', 23: '赞', 24: '赞', 25: '赞', 26: '赞', 27: '赞', 28: '赞', 29: '赞', 30: '赞', 31: '赞'}
 >>> for each in dict1.keys():
 ...     print(each, end = ' ')
 ...
 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 >>>
 >>> for each in dict1.values():
 ...     print(each, end = ' ')
 ...
 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 赞 >>>
 >>> for each in dict1.items():
 ...     print(each, end = ' ')
 ...
 (0, '赞') (1, '赞') (2, '赞') (3, '赞') (4, '赞') (5, '赞') (6, '赞') (7, '赞') (8, '赞') (9, '赞') (10, '赞') (11, '赞') (12, '赞') (13, '赞') (14, '赞') (15, '赞') (16, '赞') (17, '赞') (18, '赞') (19, '赞') (20, '赞') (21, '赞') (22, '赞') (23, '赞') (24, '赞') (25, '赞') (26, '赞') (27, '赞') (28, '赞') (29, '赞') (30, '赞') (31, '赞') >>>
 >>> dict1.keys()
 dict_keys([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31])
 >>> dict1.values()
 dict_values(['赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞', '赞'])
 >>> dict1.items()
 dict_items([(0, '赞'), (1, '赞'), (2, '赞'), (3, '赞'), (4, '赞'), (5, '赞'), (6, '赞'), (7, '赞'), (8, '赞'), (9, '赞'), (10, '赞'), (11, '赞'), (12, '赞'), (13, '赞'), (14, '赞'), (15, '赞'), (16, '赞'), (17, '赞'), (18, '赞'), (19, '赞'), (20, '赞'), (21, '赞'), (22, '赞'), (23, '赞'), (24, '赞'), (25, '赞'), (26, '赞'), (27, '赞'), (28, '赞'), (29, '赞'), (30, '赞'), (31, '赞')])
 >>> 

使用不存在的键访问一个字典时，会引发一个KeyError异常。

>>> dict1[32]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 32

这时，可以使用get方法。

>>> dict1.get(32)
>>> dict1.get(32,'no exist')
'no exist'

可以通过成员关系操作符（in，not in）来判断一个键是否在字典中

>>> 31 in dict1
True
>>> 32 in dict2
False

字典的成员操作比序列更加高效，序列时通过不断迭代来判断的，而字典时基于hash的查找。

清空字典：clear()

>>> dict1
{0: '赞', 1: '赞', 2: '赞', 3: '赞', 4: '赞', 5: '赞', 6: '赞', 7: '赞', 8: '赞', 9: '赞', 10: '赞', 11: '赞', 12: '赞', 13: '赞', 14: '赞', 15: '赞', 16: '赞', 17: '赞', 18: '赞', 19: '赞', 20: '赞', 21: '赞', 22: '赞', 23: '赞', 24: '赞', 25: '赞', 26: '赞', 27: '赞', 28: '赞', 29: '赞', 30: '赞', 31: '赞'}
>>> dict2 = dict1
>>> dict1 = {}
>>> dict1
{}
>>> dict2
{0: '赞', 1: '赞', 2: '赞', 3: '赞', 4: '赞', 5: '赞', 6: '赞', 7: '赞', 8: '赞', 9: '赞', 10: '赞', 11: '赞', 12: '赞', 13: '赞', 14: '赞', 15: '赞', 16: '赞', 17: '赞', 18: '赞', 19: '赞', 20: '赞', 21: '赞', 22: '赞', 23: '赞', 24: '赞', 25: '赞', 26: '赞', 27: '赞', 28: '赞', 29: '赞', 30: '赞', 31: '赞'}
>>> dict1 = dict2
>>> dict1.clear()
>>> dict1
{}
>>> dict2
{}

字典拷贝：copy()

　　这是浅拷贝，和直接赋值不同。浅拷贝，深拷贝都是对一个对象的浅层或深层的拷贝，但是，直接赋值则只是多了一个别名。

>>> a = {1:'one',2:'two',3:'three'}
>>> b = a.copy()
>>> c = a

　　a，c是同一个对象的两个不同的名字，修改其中任意一个都会是另一个的值发生改变，因为本质上修改对方和修改自己是一样的。而b则是另一个不同的对象，对a或c作任何改变都不会印象b，反过来亦是如此。

pop() 给出键，弹出值

popitem() 随机弹出一个项

>>> a.pop(2)
'two'
>>> c
{1: 'one', 3: 'three'}
>>> c.popitem()
(1, 'one')
>>> b
{1: 'one', 2: 'two', 3: 'three'}

setdefault() 和get类似，但找不到键时进行添加

>>> a.setdefault('white')
>>> a
{'white': None, 3: 'three'}
>>> a.setdefault('white',5)
>>> a
{'white': None, 3: 'three'}
>>> a.setdefault('black',5)
5
>>> a
{'white': None, 'black': 5, 3: 'three'}
>>> a.setdefault('black',5)
5
>>> a.setdefault('black',6)
5
>>> a
{'white': None, 'black': 5, 3: 'three'}

update() 利用一个字典或映射关系去更新一个字典

>>> a
{'white': None, 'black': 5, 3: 'three'}
>>> b
{1: 'one', 2: 'two', 3: 'three'}
>>> a.update(b)
>>> a
{1: 'one', 2: 'two', 3: 'three', 'white': None, 'black': 5}