python

下面介绍字典采用的主流方法。

1、items()方法

字典的items()方法用于返回该字典所有键-值对的字典视图：

1. >>> someone = {'name': 'Wang', 'age': 19, 'hobbies': ['sing songs', 'dance', 'basketball']}
2. >>> someone.items()
3. dict_items([('name', 'Wang'), ('age', 19), ('hobbies', ['sing songs', 'dance', 'basketball'])])

字典所有键-值对的字典视图同样可以迭代：

1. >>> someone = {'name': 'Wang', 'age': 19, 'hobbies': ['sing songs', 'dance', 'basketball']}
2. >>> for item in someone.items():
3. ... print(item)
4. ...
5. ('name', 'Wang')
6. ('age', 19)
7. ('hobbies', ['sing songs', 'dance', 'basketball'])

也可以使用类似嵌套元组解包的形式进行迭代：

1. >>> for k, v in someone.items():
2. ... print('{0} -> {1}'.format(k, v)) # 使用format()格式化
3. ...
4. name -> Wang
5. age -> 19
6. hobbies -> ['sing songs', 'dance', 'basketball']

可以利用items()方法在迭代字典时修改该字典：

1. >>> grades = {'math': 69, 'Chinese': 78, 'English': 83}
2. >>> for k, v in grades.items():
3. ... grades[k] = v - 10 # 将该学生每科成绩都减了10分
4. ...
5. >>> grades
6. {'math': 59, 'Chinese': 68, 'English': 73}

2、update()方法

字典的update()方法使用另一个字典对象或可迭代的键-值对更新字典。如果键不在字典中，那么update()方法将项添加到字典中：

1. >>> grades = {'math': 69, 'Chinese': 78, 'English': 83}
2. >>> other = {'history': 96, 'physics': 56} # 将使用该字典更新grades字典
3. >>> grades.update(other)
4. >>> grades # grades字典中新增了两个键-值对
5. {'math': 69, 'Chinese': 78, 'English': 83, 'history': 96, 'physics': 56}
6. >>> other # other字典并没有改变
7. {'history': 96, 'physics': 56}

如果键在字典中，那么update()方法将使用新值更新键：

1. >>> grades = {'math': 69, 'Chinese': 78, 'English': 83}
2. >>> other = {'history': 96, 'English': 56}
3. >>> grades.update(other)
4. >>> grades # 键'English'的值更新了
5. {'math': 69, 'Chinese': 78, 'English': 56, 'history': 96}

3、pop()方法

前文提到过，字典的pop()方法可以移除指定键并返回该键对应的值，但如果指定的键不在字典中，Python解释器会报错：

1. >>> someone = {'name': 'Wang', 'age': 19, 'hobbies': ['sing songs', 'dance', 'basketball']}
2. >>> someone.pop('gender') # 键'gender'并不存在
3. Traceback (most recent call last):
4. File "<stdin>", line 1, in <module>
5. KeyError: &#39;gender&#39;

pop()方法可以提供一个当指定键不存在时的默认值：

1. >>> someone = {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}
2. >>> gender = someone.pop(&#39;gender&#39;, &#39;famale&#39;)
3. >>> gender
4. &#39;famale&#39;
5. >>> someone # 此时someone字典并没有改变
6. {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}

4、popitem()方法

字典的popitem()方法和列表的pop()方法很像，都是“弹出”并返回一个元素。在Python 3.5及之前的版本中，字典是无序的，因此，popitem()是随机弹出一个项;但从Python 3.6开始，字典是按插入的顺序排列的，因此，popitem()弹出的是最后一个项。

首先验证一下Python 3.5和3.6这两个版本的有序性(Python 2和这两个版本的表现也均不一致，本书不详细探讨)。在Python 3.5中：

1. >>> someone = {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}
2. >>> someone # 查看someone
3. {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}
4. >>> someone # 再次查看someone，发现它并没有改变
5. {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}

然后关闭Python交互模式，再重新进入Python交互模式：

1. >>> someone = {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}
2. >>> someone # someone字典键-值对的顺序发生了改变
3. {&#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;], &#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19}
4. >>> someone
5. {&#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;], &#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19}

由于在Python 3.5及之前的版本中，字典是无序的，因此，上面的情况可能会出现(由于是随机的，当然也可能顺序不变)。但是从Python 3.6开始，执行和前面一样的操作，无论做多少次，都会得到下面的结果：

1. >>> someone = {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}
2. >>> someone
3. {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}
4. >>> someone
5. {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}

确认Python 3.6中字典是有序的之后，尝试使用popitems()方法：

1. >>> someone = {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}
2. >>> someone.popitem() # 弹出并返回最后一个项
3. (&#39;hobbies&#39;, [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;])
4. >>> someone.popitem()
5. (&#39;age&#39;, 19)
6. >>> someone.popitem()
7. (&#39;name&#39;, &#39;Wang&#39;)
8. >>> someone.popitem() # 如果字典为空，Python解释器会报错
9. Traceback (most recent call last):
10. File "<stdin>", line 1, in <module>
11. KeyError: &#39;popitem(): dictionary is empty&#39;

5、copy()方法

与列表的copy()方法类似，字典的copy()方法返回一个和被拷贝字典相同的字典(同样是浅复制)：

1. >>> someone = {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}
2. >>> other = someone.copy()
3. >>> other
4. {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}
5. >>> other[&#39;gender&#39;] = &#39;female&#39; # other字典中添加一个键-值对
6. >>> other # other字典改变了
7. {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;], &#39;gender&#39;: &#39;female&#39;}
8. >>> someone # someone字典并没有改变
9. {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}

6、fromkeys()方法

字典的fromkeys()方法用于根据指定序列创建一个新字典，该字典具有用户提供的值。fromkeys()方法返回一个新字典，其中指定序列中的项作为字典的键。如果设置了value参数，那么字典所有键对应的值都将设置为提供的值;如果未设置，那么默认值为None：

1. >>> someone = {}.fromkeys([&#39;name&#39;, &#39;age&#39;, &#39;hobbies&#39;]) # 默认值为None
2. >>> someone
3. {&#39;name&#39;: None, &#39;age&#39;: None, &#39;hobbies&#39;: None}
4. >>> someone = {}.fromkeys([&#39;name&#39;, &#39;age&#39;, &#39;hobbies&#39;], &#39;a&#39;) # 指定默认值
5. >>> someone
6. {&#39;name&#39;: &#39;a&#39;, &#39;age&#39;: &#39;a&#39;, &#39;hobbies&#39;: &#39;a&#39;}

7、clear()方法

字典的clear()方法用于清除字典中的所有项：

1. >>> someone = {&#39;name&#39;: &#39;Wang&#39;, &#39;age&#39;: 19, &#39;hobbies&#39;: [&#39;sing songs&#39;, &#39;dance&#39;, &#39;basketball&#39;]}
2. >>> someone.clear()
3. >>> someone # 此时someone字典是一个空字典
4. {}