130 lines
3.8 KiB
Markdown
130 lines
3.8 KiB
Markdown
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---
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title: The Python Dict
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localeTitle: Python Dict
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---
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python中的Dictionary（又名“dict”）是一种内置数据类型，可用于存储**`key-value`**对。这允许您将**`dict`**视为存储和组织数据的_数据库_ 。
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关于字典的特殊之处在于它们的实现方式。类似哈希表的结构使其易于检查 存在 - 这意味着我们可以轻松确定字典中是否存在特定键而无需检查 每个元素。 Python解释器只需转到位置键并检查密钥是否存在。
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字典几乎可以使用任意数据类型，如字符串，整数等，用于键。但是，不可清除的值， 也就是说，包含列表，字典或其他可变类型（通过值而不是通过对象标识进行比较）的值不能用作键。用于键的数字类型遵循用于数字比较的常规规则：如果两个数字比较相等（例如`1`和`1.0` ），则它们可以互换地用于索引相同的字典条目。 （但请注意，由于计算机将浮点数存储为近似值，因此将它们用作字典键通常是不明智的。）
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字典的一个最重要的要求是密钥**必须**是唯一的。
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要创建一个空字典，只需使用一对大括号：
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```python
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>>> teams = {}
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>>> type(teams)
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>>> <class 'dict'>
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```
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要创建包含一些初始值的非空字典，请放置以逗号分隔的键值对列表：
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```python
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>>> teams = {'barcelona': 1875, 'chelsea': 1910}
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>>> teams
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{'barcelona': 1875, 'chelsea': 1910}
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```
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将键值对添加到现有字典很容易：
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```python
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>>> teams['santos'] = 1787
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>>> teams
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{'chelsea': 1910, 'barcelona': 1875, 'santos': 1787} # Notice the order - Dictionaries are unordered !
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>>> # extracting value - Just provide the key
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...
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>>> teams['barcelona']
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1875
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```
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**`del`**运算符用于从dict中删除键值对。在已经使用的密钥再次用于存储值的情况下，与该密钥关联的旧值完全丢失。另外，请记住，使用不存在的密钥提取值是错误的。
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```python
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>>> del teams['santos']
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>>> teams
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{'chelsea': 1910, 'barcelona': 1875}
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>>> teams['chelsea'] = 2017 # overwriting
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>>> teams
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{'chelsea': 2017, 'barcelona': 1875}
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```
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**`in`** keyword可用于检查dict中是否存在密钥：
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```python
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>>> 'sanots' in teams
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False
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>>> 'barcelona' in teams
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True
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>>> 'chelsea' not in teams
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False
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```
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**`keys`**是一种内置_方法_ ，可用于获取给定字典的键。要将dict中存在的键提取为列表：
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```python
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>>> club_names = list(teams.keys())
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>>> club_names
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['chelsea', 'barcelona']
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```
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另一种创建字典的方法是使用**`dict()`**方法：
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```python
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>>> players = dict( [('messi','argentina'), ('ronaldo','portugal'), ('kaka','brazil')] ) # sequence of key-value pair is passed
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>>> players
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{'ronaldo': 'portugal', 'kaka': 'brazil', 'messi': 'argentina'}
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>>>
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>>> # If keys are simple strings, it's quite easier to specify pairs using keyword arguments
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...
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>>> dict( totti = 38, zidane = 43 )
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{'zidane': 43, 'totti': 38}
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```
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也可以使用Dict理解来从任意键和值表达式创建字典：
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```python
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>>> {x: x**2 for x in (2, 4, 6)}
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{2: 4, 4: 16, 6: 36}
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```
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**在词典中循环**
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简单地循环遍历字典中的键，而不是键和值：
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```python
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>>> d = {'x': 1, 'y': 2, 'z': 3}
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>>> for key in d:
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... print(key) # do something
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...
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x
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y
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z
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```
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要循环键和值，您可以使用以下内容：
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对于Python 2.x：
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```python
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>>> for key, item in d.iteritems():
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... print items
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...
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1
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2
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3
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```
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对Python 3.x使用**`items()`** ：
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```python
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>>> for key, item in d.items():
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... print(key, items)
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...
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x 1
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y 2
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z 3
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```
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