说明：
显然Python中字典的学习过程与列表是一样的，主要是围绕下面的函数来进行重点学习：
>>> xpleaf. xpleaf.clear( xpleaf.copy( xpleaf.get( xpleaf.has_key( xpleaf.items( xpleaf.keys( xpleaf.pop( xpleaf.popitem( xpleaf.setdefault( xpleaf.update(
1.基本操作
--创建一个字典
>>> xpleaf = {
... ‘name‘:‘xpleaf‘,
... ‘occupation‘:‘student‘,
... ‘hobby‘:‘computer‘,
... ‘dream‘:‘excellent hacker‘
... }
>>> xpleaf
{‘hobby‘: ‘computer‘, ‘dream‘: ‘excellent hacker‘, ‘name‘: ‘xpleaf‘, ‘occupation‘: ‘student‘}
·容易观察到字典的输出并没有按照创建字典时的顺序进行输出，因为字典按哈希值查找内容，而不是按索引号；
·{key:value}是字典的基本语法格式，key是唯一的，value可为大多数数据类型；
--查看键值对应的内容
>>> xpleaf[‘hobby‘] ‘computer‘
--修改键值对应的内容
>>> xpleaf[‘hobby‘] = ‘IT‘
>>> xpleaf
{‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘name‘: ‘xpleaf‘, ‘occupation‘: ‘student‘}
--添加一个键值对
>>> xpleaf[‘girlfriend‘] = ‘none‘
>>> xpleaf
{‘girlfriend‘: ‘none‘, ‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘name‘: ‘xpleaf‘, ‘occupation‘: ‘student‘}
·添加的元素在字典中的排序是随机的，因为索引号对字典没有意义（按照哈希值进行value值的查找）；
2.has_key()函数
·功能：接受key的查询，以bool值形式返回查询字典中是否有该key；
·演示如下：
>>> xpleaf.has_key(‘dream‘) True >>> xpleaf.has_key(‘wife‘) False
3.items()函数
·功能：将字典转换为列表，列表的元素为元组，其中左元素为key，右元素为value；
·演示如下：
>>> xpleaf.items() [(‘girlfriend‘, ‘none‘), (‘hobby‘, ‘IT‘), (‘dream‘, ‘excellent hacker‘), (‘name‘, ‘xpleaf‘), (‘occupation‘, ‘student‘)]
·基于上述输出形式，可对字典的key和value进行遍历，如下：
>>> for key,value in xpleaf.items(): ... print key,value ... girlfriend none hobby IT dream excellent hacker name xpleaf occupation student
·item()函数的原理是把字典转换为列表存储在内存中，对于数据量大的情况下，会比较慢；
·大数据量的字典遍历方法：
>>> for key in xpleaf: ... print key,xpleaf[key] ... girlfriend none hobby IT dream excellent hacker name xpleaf occupation student
4.get()函数
·功能：取对应key的value值；
·演示如下：
>>> xpleaf
{‘girlfriend‘: ‘none‘, ‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘name‘: ‘xpleaf‘, ‘occupation‘: ‘student‘}
>>> xpleaf.get(‘dream‘)
‘excellent hacker‘
>>> xpleaf.get(‘wife‘) ===>如果没有该key值则不会有输出
>>>
·即相当于dict[key]的方法取value值；
5.keys()函数
·功能：取出字典中的key值，并生成相应的列表；
·演示如下：
>>> xpleaf.keys() [‘girlfriend‘, ‘hobby‘, ‘dream‘, ‘name‘, ‘occupation‘]
5.pop()函数
·功能：弹出一个key，即删除一个键值对；
·演示如下：
>>> xpleaf
{‘girlfriend‘: ‘none‘, ‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘name‘: ‘xpleaf‘, ‘occupation‘: ‘student‘}
>>> xpleaf.pop(‘girlfriend‘)
‘none‘
>>> xpleaf
{‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘name‘: ‘xpleaf‘, ‘occupation‘: ‘student‘}
6.popitem()函数
·功能：按顺序删除字典中的元素；
·演示如下：
>>> a
{‘a‘: 1, ‘c‘: 3, ‘b‘: 2, ‘e‘: 5, ‘d‘: 4, 6: ‘f‘}
>>> a.popitem()
(‘a‘, 1)
>>> a.popitem()
(‘c‘, 3)
>>> a.popitem()
(‘b‘, 2)
>>> a.popitem()
(‘e‘, 5)
>>> a.popitem()
(‘d‘, 4)
>>> a.popitem()
(6, ‘f‘)
7.setdefault()函数
·在字典中添加元素，如果原来存在该元素，则不进行任何修改；
·演示如下：
>>> xpleaf
{‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘name‘: ‘xpleaf‘, ‘occupation‘: ‘student‘}
>>> xpleaf.setdefault(‘hobby‘,‘computer‘) ===>‘hobby‘键值对已经存在，不会添加
‘IT‘
>>> xpleaf
{‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘name‘: ‘xpleaf‘, ‘occupation‘: ‘student‘}
>>> xpleaf.setdefault(‘weight‘,‘55kg‘) ===>‘weight‘键值对不存在，会进行添加
‘55kg‘
>>> xpleaf
{‘name‘: ‘xpleaf‘, ‘weight‘: ‘55kg‘, ‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘occupation‘: ‘student‘}
>>> xpleaf.setdefault(‘wife‘) ===>添加没有的键值对，
>>> xpleaf
{‘name‘: ‘xpleaf‘, ‘weight‘: ‘55kg‘, ‘wife‘: None, ‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘occupation‘: ‘student‘}
8.update()函数
·功能：合并两个字典
·演示如下：
>>> a
{‘a‘: 1, ‘c‘: 3, ‘b‘: 2}
>>> b
{‘e‘: 4, ‘g‘: 6, ‘f‘: 5}
>>> a.update(b)
>>> a
{‘a‘: 1, ‘c‘: 3, ‘b‘: 2, ‘e‘: 4, ‘g‘: 6, ‘f‘: 5}
>>> b
{‘e‘: 4, ‘g‘: 6, ‘f‘: 5}
·合并的顺序依然是随机的，原理与前面一样；
·更新的只是字典a，字典b没有变化；
·如果合并字典时有重复的item项，则会进行覆盖：
>>> a
{‘a‘: 1, ‘c‘: 3, ‘b‘: 2, ‘e‘: 4, ‘g‘: 6, ‘f‘: 5}
>>> c
{‘b‘: ‘cover2‘, ‘g‘: ‘cover1‘}
>>> a.update(c)
>>> a
{‘a‘: 1, ‘c‘: 3, ‘b‘: ‘cover2‘, ‘e‘: 4, ‘g‘: ‘cover1‘, ‘f‘: 5}
9.values()函数
·功能：取字典中所有key的value值，并生成相应的列表
·演示如下：
>>> xpleaf
{‘name‘: ‘xpleaf‘, ‘weight‘: ‘55kg‘, ‘wife‘: None, ‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘occupation‘: ‘student‘}
>>> xpleaf.values()
[‘xpleaf‘, ‘55kg‘, None, ‘IT‘, ‘excellent hacker‘, ‘student‘]
·多用在value值的数据类型都相同的字典中，以用于数据的批量分析；
10.clear()函数
·功能：清空字典的item项
·演示如下：
>>> a
{‘a‘: 1, ‘c‘: 3, ‘b‘: ‘cover2‘, ‘e‘: 4, ‘g‘: ‘cover1‘, ‘f‘: 5}
>>> a.clear()
>>> a
{}
·与del不同，del是直接删除字典：
>>> del a >>> a Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name ‘a‘ is not defined
11.copy()函数
·功能：对字典进行浅复制；
·Python中普通情况下的“复制”：
>>> xpleaf
{‘name‘: ‘xpleaf‘, ‘weight‘: ‘55kg‘, ‘wife‘: None, ‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘occupation‘: ‘student‘}
>>> xpleaf_copy = xpleaf
>>> xpleaf_copy
{‘name‘: ‘xpleaf‘, ‘weight‘: ‘55kg‘, ‘wife‘: None, ‘hobby‘: ‘IT‘, ‘dream‘: ‘excellent hacker‘, ‘occupation‘: ‘student‘}
>>> xpleaf[‘hobby‘] = ‘IT_Field‘
>>> xpleaf_copy
{‘name‘: ‘xpleaf‘, ‘weight‘: ‘55kg‘, ‘wife‘: None, ‘hobby‘: ‘IT_Field‘, ‘dream‘: ‘excellent hacker‘, ‘occupation‘: ‘student‘}
>>> xpleaf_copy[‘wife‘] = ‘None!!!‘
>>> xpleaf_copy
{‘name‘: ‘xpleaf‘, ‘weight‘: ‘55kg‘, ‘wife‘: ‘None!!!‘, ‘hobby‘: ‘IT_Field‘, ‘dream‘: ‘excellent hacker‘, ‘occupation‘: ‘student‘}
>>> xpleaf
{‘name‘: ‘xpleaf‘, ‘weight‘: ‘55kg‘, ‘wife‘: ‘None!!!‘, ‘hobby‘: ‘IT_Field‘, ‘dream‘: ‘excellent hacker‘, ‘occupation‘: ‘student‘}
·即将变量赋给其它变量只是将对象（实际的字典）作一个引用传递而已，修改任何一个引用都会改变原来对象的值；
·copy()的浅复制功能则不是引用传递：
>>> xpleaf_copy2 = xpleaf.copy()
>>> xpleaf_copy2
{‘name‘: ‘xpleaf‘, ‘weight‘: ‘55kg‘, ‘wife‘: ‘None!!!‘, ‘hobby‘: ‘IT_Field‘, ‘dream‘: ‘excellent hacker‘, ‘occupation‘: ‘student‘}
>>> xpleaf_copy2[‘wife‘] = ‘CL‘
>>> xpleaf_copy2
{‘name‘: ‘xpleaf‘, ‘weight‘: ‘55kg‘, ‘wife‘: ‘CL‘, ‘hobby‘: ‘IT_Field‘, ‘dream‘: ‘excellent hacker‘, ‘occupation‘: ‘student‘}
>>> xpleaf
{‘name‘: ‘xpleaf‘, ‘weight‘: ‘55kg‘, ‘wife‘: ‘None!!!‘, ‘hobby‘: ‘IT_Field‘, ‘dream‘: ‘excellent hacker‘, ‘occupation‘: ‘student‘}
·当然copy()更重要的作用不仅在于此，这里只是简单提及它的作用。
时间： 2024-10-26 21:06:28