5.5 字典 Dictionaries

Another useful data type built into Python is the dictionary. Dictionaries are sometimes found in other languages as ``associative memories'' or ``associative arrays''. Unlike sequences, which are indexed by a range of numbers, dictionaries are indexed by keys, which can be any immutable type; strings and numbers can always be keys. Tuples can be used as keys if they contain only strings, numbers, or tuples; if a tuple contains any mutable object either directly or indirectly, it cannot be used as a key. You can't use lists as keys, since lists can be modified in place using their append() and extend() methods, as well as slice and indexed assignments.

另一个非常有用的Python内建数据类型是字典。字典在某些语言中可能称为“联合内存”（``associative memories''）或“联合数组”（``associative arrays''）。序列是以连续的整数为索引，与此不同的是，字典以关键字为索引，关键字可以是任意不可变类型，通常用字符串或数值。如果元组中只包含字符串和数字，它可以做为关键字，如果它直接或间接的包含了可变对象，就不能当做关键字。不能用链表做关键字，因为链表可以用它们的 append() 和 extend() 方法，或者用切片、或者通过检索变量来即时改变。

It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary). A pair of braces creates an empty dictionary: {}. Placing a comma-separated list of key:value pairs within the braces adds initial key:value pairs to the dictionary; this is also the way dictionaries are written on output.

理解字典的最佳方式是把它看做无序的关键字：值 对（ key:value pairs ）集合，关键字必须是互不相同的（在同一个字典之内）。一对大括号创建一个空的字典：{}。初始化链表时，在大括号内放置一组逗号分隔的关键字：值对，这也是字典输出的方式。

The main operations on a dictionary are storing a value with some key and extracting the value given the key. It is also possible to delete a key:value pair with del. If you store using a key that is already in use, the old value associated with that key is forgotten. It is an error to extract a value using a non-existent key.

字典的主要操作是依据关键字来存储和析取值。也可以用 del 来删除关键字：值对。如果你用一个已经存在的关键字存储值，以前为该关键字分配的值就会被遗忘。试图析取从一个不存在的关键字中读取值会导致错误。

The keys() method of a dictionary object returns a list of all the keys used in the dictionary, in random order (if you want it sorted, just apply the sort() method to the list of keys). To check whether a single key is in the dictionary, use the has_key() method of the dictionary.

字典的 keys() 方法返回由所有关键字组成的链表，该链表的顺序不定（如果你需要它有序，只能调用关键字链表的 sort() 方法）。使用字典的 has_key() 方法可以检查字典中是否存在某一关键字。

>>> tel = {'jack': 4098, 'sape': 4139}
>>> tel['guido'] = 4127
>>> tel
{'sape': 4139, 'guido': 4127, 'jack': 4098}
>>> tel['jack']
4098
>>> del tel['sape']
>>> tel['irv'] = 4127
>>> tel
{'guido': 4127, 'irv': 4127, 'jack': 4098}
>>> tel.keys()
['guido', 'irv', 'jack']
>>> tel.has_key('guido')
True

The dict() constructor builds dictionaries directly from lists of key-value pairs stored as tuples. When the pairs form a pattern, list comprehensions can compactly specify the key-value list.

链表中存储关键字-值对元组的话，字典可以从中直接构造。关键字-值对来自一个模式时，可以用链表推导式简单的表达关键字-值链表。

>>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
{'sape': 4139, 'jack': 4098, 'guido': 4127}
>>> dict([(x, x**2) for x in vec])     # use a list comprehension
{2: 4, 4: 16, 6: 36}