8.3. collections — 高性能容器数据类型 ¶

>>> Account = namedtuple('Account', 'owner balance transaction_count')
>>> default_account = Account('<owner name>', 0.0, 0)
>>> johns_account = default_account._replace(owner='John')
																									

>>> Status = namedtuple('Status', 'open pending closed')._make(range(3))
>>> Status.open, Status.pending, Status.closed
(0, 1, 2)
>>> class Status:
...     open, pending, closed = range(3)
																										

另请参阅

Named tuple recipe adapted for Python 2.4.

8.3.5. OrderedDict 对象 ¶

Ordered dictionaries are just like regular dictionaries but they remember the order that items were inserted. When iterating over an ordered dictionary, the items are returned in the order their keys were first added.

class collections. OrderedDict ( [ 项 ] ) ¶

返回 dict 子类实例，支持通常 dict 方法。 OrderedDict is a dict that remembers the order that keys were first inserted. If a new entry overwrites an existing entry, the original insertion position is left unchanged. Deleting an entry and reinserting it will move it to the end.

2.7 版新增。

OrderedDict. popitem ( last=True ) ¶

The popitem() method for ordered dictionaries returns and removes a (key, value) pair. The pairs are returned in LIFO order if last is true or FIFO order if false.

除通常映射方法外，有序词典还支持反向迭代使用 reversed() .

相等性测试在 OrderedDict 对象是次序敏感的，且被实现为 list(od1.items())==list(od2.items()) 。相等性测试在 OrderedDict 对象和其它 Mapping 对象像常规字典是次序不敏感的。这允许 OrderedDict 对象要在任何地方使用常规字典被代入。

The OrderedDict constructor and update() method both accept keyword arguments, but their order is lost because Python’s function call semantics pass-in keyword arguments using a regular unordered dictionary.

另请参阅

Equivalent OrderedDict recipe that runs on Python 2.4 or later.

8.3.5.1. OrderedDict 范例和配方 ¶

Since an ordered dictionary remembers its insertion order, it can be used in conjunction with sorting to make a sorted dictionary:

>>> # regular unsorted dictionary
>>> d = {'banana': 3, 'apple': 4, 'pear': 1, 'orange': 2}
>>> # dictionary sorted by key
>>> OrderedDict(sorted(d.items(), key=lambda t: t[0]))
OrderedDict([('apple', 4), ('banana', 3), ('orange', 2), ('pear', 1)])
>>> # dictionary sorted by value
>>> OrderedDict(sorted(d.items(), key=lambda t: t[1]))
OrderedDict([('pear', 1), ('orange', 2), ('banana', 3), ('apple', 4)])
>>> # dictionary sorted by length of the key string
>>> OrderedDict(sorted(d.items(), key=lambda t: len(t[0])))
OrderedDict([('pear', 1), ('apple', 4), ('orange', 2), ('banana', 3)])
																										

The new sorted dictionaries maintain their sort order when entries are deleted. But when new keys are added, the keys are appended to the end and the sort is not maintained.

It is also straight-forward to create an ordered dictionary variant that remembers the order the keys were last inserted. If a new entry overwrites an existing entry, the original insertion position is changed and moved to the end:

class LastUpdatedOrderedDict(OrderedDict):
    'Store items in the order the keys were last added'
    def __setitem__(self, key, value):
        if key in self:
            del self[key]
        OrderedDict.__setitem__(self, key, value)
																										

An ordered dictionary can be combined with the Counter class so that the counter remembers the order elements are first encountered:

class OrderedCounter(Counter, OrderedDict):
     'Counter that remembers the order elements are first encountered'
     def __repr__(self):
         return '%s(%r)' % (self.__class__.__name__, OrderedDict(self))
     def __reduce__(self):
         return self.__class__, (OrderedDict(self),)
																										

8.3.6. 集合抽象基类 ¶

The collections module offers the following ABCs :

ABC (抽象基类)	Inherits from	Abstract Methods	Mixin Methods
Container		__contains__
Hashable		__hash__
Iterable		__iter__
Iterator	Iterable	next	__iter__
Sized		__len__
Callable		__call__
Sequence	Sized , Iterable , Container	__getitem__ , __len__	__contains__ , __iter__ , __reversed__ , index ，和 count
MutableSequence	Sequence	__getitem__ , __setitem__ , __delitem__ , __len__ , insert	继承 Sequence methods and append , reverse , extend , pop , remove ，和 __iadd__
Set	Sized , Iterable , Container	__contains__ , __iter__ , __len__	__le__ , __lt__ , __eq__ , __ne__ , __gt__ , __ge__ , __and__ , __or__ , __sub__ , __xor__ ，和 isdisjoint
MutableSet	Set	__contains__ , __iter__ , __len__ , add , discard	继承 Set methods and clear , pop , remove , __ior__ , __iand__ , __ixor__ ，和 __isub__
Mapping	Sized , Iterable , Container	__getitem__ , __iter__ , __len__	__contains__ , keys , items , values , get , __eq__ ，和 __ne__
MutableMapping	Mapping	__getitem__ , __setitem__ , __delitem__ , __iter__ , __len__	继承 Mapping methods and pop , popitem , clear , update ，和 setdefault
MappingView	Sized		__len__
ItemsView	MappingView , Set		__contains__ , __iter__
KeysView	MappingView , Set		__contains__ , __iter__
ValuesView	MappingView		__contains__ , __iter__

class collections. Container ¶

class collections. Hashable ¶

class collections. Sized ¶

class collections. 可调用 ¶

ABCs for classes that provide respectively the methods __contains__() , __hash__() , __len__() ，和 __call__() .

class collections. Iterable ¶

ABC for classes that provide the __iter__() method. See also the definition of iterable .

class collections. 迭代器 ¶

ABC for classes that provide the __iter__() and next() methods. See also the definition of iterator .

class collections. Sequence ¶

class collections. MutableSequence ¶

ABCs for read-only and mutable sequences .

class collections. Set ¶

class collections. MutableSet ¶

ABCs for read-only and mutable sets.

class collections. 映射 ¶

class collections. MutableMapping ¶

ABCs for read-only and mutable mappings .

class collections. MappingView ¶

class collections. ItemsView ¶

class collections. KeysView ¶

class collections. ValuesView ¶

ABCs for mapping, items, keys, and values views .

These ABCs allow us to ask classes or instances if they provide particular functionality, for example:

size = None
if isinstance(myvar, collections.Sized):
    size = len(myvar)
																										

Several of the ABCs are also useful as mixins that make it easier to develop classes supporting container APIs. For example, to write a class supporting the full Set API, it only necessary to supply the three underlying abstract methods: __contains__() , __iter__() ，和 __len__() . The ABC supplies the remaining methods such as __and__() and isdisjoint()

class ListBasedSet(collections.Set):
     ''' Alternate set implementation favoring space over speed
         and not requiring the set elements to be hashable. '''
     def __init__(self, iterable):
         self.elements = lst = []
         for value in iterable:
             if value not in lst:
                 lst.append(value)
     def __iter__(self):
         return iter(self.elements)
     def __contains__(self, value):
         return value in self.elements
     def __len__(self):
         return len(self.elements)
s1 = ListBasedSet('abcdef')
s2 = ListBasedSet('defghi')
overlap = s1 & s2            # The __and__() method is supported automatically
																										

Notes on using Set and MutableSet as a mixin:

1. Since some set operations create new sets, the default mixin methods need a way to create new instances from an iterable. The class constructor is assumed to have a signature in the form ClassName(iterable) . That assumption is factored-out to an internal classmethod called _from_iterable() 调用 cls(iterable) to produce a new set. If the Set mixin is being used in a class with a different constructor signature, you will need to override _from_iterable() with a classmethod that can construct new instances from an iterable argument.

2. To override the comparisons (presumably for speed, as the semantics are fixed), redefine __le__() and __ge__() , then the other operations will automatically follow suit.

3. The Set mixin provides a _hash() method to compute a hash value for the set; however, __hash__() is not defined because not all sets are hashable or immutable. To add set hashability using mixins, inherit from both Set() and Hashable() , then define __hash__ = Set._hash .

另请参阅

• OrderedSet recipe for an example built on MutableSet .

• For more about ABCs, see the abc module and PEP 3119 .