collections.abc --- 容器的抽象基類?

3.3 新版功能: 該模塊曾是 collections 模塊的組成部分。

源代碼： Lib/_collections_abc.py

該模塊定義了一些 抽象基類，它們可用于判斷一個(gè)具體類是否具有某一特定的接口；例如，這個(gè)類是否可哈希，或其是否為映射類。

An issubclass() or isinstance() test for an interface works in one of three ways.

1) A newly written class can inherit directly from one of the abstract base classes. The class must supply the required abstract methods. The remaining mixin methods come from inheritance and can be overridden if desired. Other methods may be added as needed:

class C(Sequence):                      # Direct inheritance
    def __init__(self): ...             # Extra method not required by the ABC
    def __getitem__(self, index):  ...  # Required abstract method
    def __len__(self):  ...             # Required abstract method
    def count(self, value): ...         # Optionally override a mixin method
>>>
>>> issubclass(C, Sequence)
True
>>> isinstance(C(), Sequence)
True

2) Existing classes and built-in classes can be registered as "virtual subclasses" of the ABCs. Those classes should define the full API including all of the abstract methods and all of the mixin methods. This lets users rely on issubclass() or isinstance() tests to determine whether the full interface is supported. The exception to this rule is for methods that are automatically inferred from the rest of the API:

class D:                                 # No inheritance
    def __init__(self): ...              # Extra method not required by the ABC
    def __getitem__(self, index):  ...   # Abstract method
    def __len__(self):  ...              # Abstract method
    def count(self, value): ...          # Mixin method
    def index(self, value): ...          # Mixin method

Sequence.register(D)                     # Register instead of inherit
>>>
>>> issubclass(D, Sequence)
True
>>> isinstance(D(), Sequence)
True

In this example, class D does not need to define __contains__, __iter__, and __reversed__ because the in-operator, the iteration logic, and the reversed() function automatically fall back to using __getitem__ and __len__.

3) Some simple interfaces are directly recognizable by the presence of the required methods (unless those methods have been set to None):

class E:
    def __iter__(self): ...
    def __next__(next): ...
>>>
>>> issubclass(E, Iterable)
True
>>> isinstance(E(), Iterable)
True

Complex interfaces do not support this last technique because an interface is more than just the presence of method names. Interfaces specify semantics and relationships between methods that cannot be inferred solely from the presence of specific method names. For example, knowing that a class supplies __getitem__, __len__, and __iter__ is insufficient for distinguishing a Sequence from a Mapping.

3.9 新版功能: These abstract classes now support []. See GenericAlias 類型 and PEP 585.

容器抽象基類?

這個(gè)容器模塊提供了以下 ABCs:

抽象基類

繼承自

抽象方法

Mixin 方法

Container 1

__contains__

Hashable 1

__hash__

Iterable 1 2

__iter__

Iterator 1

Iterable

__next__

__iter__

Reversible 1

Iterable

__reversed__

Generator 1

Iterator

send, throw

close, __iter__, __next__

Sized 1

__len__

Callable 1

__call__

Collection 1

Sized, Iterable, Container

__contains__, __iter__, __len__

Sequence

Reversible, Collection

__getitem__, __len__

__contains__, __iter__, __reversed__, index, and count

MutableSequence

Sequence

__getitem__, __setitem__, __delitem__, __len__, insert

繼承自 Sequence 的方法，以及 append, reverse, extend, pop, remove，和 __iadd__

ByteString

Sequence

__getitem__, __len__

繼承自 Sequence 的方法

Set

Collection

__contains__, __iter__, __len__

__le__, __lt__, __eq__, __ne__, __gt__, __ge__, __and__, __or__, __sub__, __xor__, and isdisjoint

MutableSet

Set

__contains__, __iter__, __len__, add, discard

繼承自 Set 的方法以及 clear, pop, remove, __ior__, __iand__, __ixor__，和 __isub__

Mapping

Collection

__getitem__, __iter__, __len__

__contains__, keys, items, values, get, __eq__, and __ne__

MutableMapping

Mapping

__getitem__, __setitem__, __delitem__, __iter__, __len__

繼承自 Mapping 的方法以及 pop, popitem, clear, update，和 setdefault

MappingView

Sized

__len__

ItemsView

MappingView, Set

__contains__, __iter__

KeysView

MappingView, Set

__contains__, __iter__

ValuesView

MappingView, Collection

__contains__, __iter__

Awaitable 1

__await__

Coroutine 1

Awaitable

send, throw

close

AsyncIterable 1

__aiter__

AsyncIterator 1

AsyncIterable

__anext__

__aiter__

AsyncGenerator 1

AsyncIterator

asend, athrow

aclose, __aiter__, __anext__

Footnotes

1(1,2,3,4,5,6,7,8,9,10,11,12,13,14)

These ABCs override object.__subclasshook__() to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

2

Checking isinstance(obj, Iterable) detects classes that are registered as Iterable or that have an __iter__() method, but it does not detect classes that iterate with the __getitem__() method. The only reliable way to determine whether an object is iterable is to call iter(obj).

Collections Abstract Base Classes -- Detailed Descriptions?

class collections.abc.Container?

提供了 __contains__() 方法的抽象基類。

class collections.abc.Hashable?

提供了 __hash__() 方法的抽象基類。

class collections.abc.Sized?

提供了 __len__() 方法的抽象基類。

class collections.abc.Callable?

提供了 __call__() 方法的抽象基類。

class collections.abc.Iterable?

提供了 __iter__() 方法的抽象基類。

使用 isinstance(obj, Iterable) 可以檢測一個(gè)類是否已經(jīng)注冊到了 Iterable 或者實(shí)現(xiàn)了 __iter__() 函數(shù)，但是無法檢測這個(gè)類是否能夠使用 __getitem__() 方法進(jìn)行迭代。檢測一個(gè)對(duì)象是否是 iterable 的唯一可信賴的方法是調(diào)用 iter(obj)。

class collections.abc.Collection?

集合了 Sized 和 Iterable 類的抽象基類。

3.6 新版功能.

class collections.abc.Iterator?

提供了 __iter__() 和 __next__() 方法的抽象基類。參見 iterator 的定義。

class collections.abc.Reversible?

為可迭代類提供了 __reversed__() 方法的抽象基類。

3.6 新版功能.

class collections.abc.Generator?

生成器類，實(shí)現(xiàn)了 PEP 342 中定義的協(xié)議，繼承并擴(kuò)展了迭代器，提供了 send(), throw() 和 close() 方法。參見 generator 的定義。

3.5 新版功能.

class collections.abc.Sequence?
class collections.abc.MutableSequence?
class collections.abc.ByteString?

只讀且可變的序列 sequences 的抽象基類。

實(shí)現(xiàn)筆記：一些混入（Maxin）方法比如 __iter__(), __reversed__() 和 index() 會(huì)重復(fù)調(diào)用底層的 __getitem__() 方法。因此，如果實(shí)現(xiàn)的 __getitem__() 是常數(shù)級(jí)訪問速度，那么相應(yīng)的混入方法會(huì)有一個(gè)線性的表現(xiàn)；然而，如果底層方法是線性實(shí)現(xiàn)（例如鏈表），那么混入方法將會(huì)是平方級(jí)的表現(xiàn)，這也許就需要被重構(gòu)了。

在 3.5 版更改: index() 方法支持 stop 和 start 參數(shù)。

class collections.abc.Set?
class collections.abc.MutableSet?

只讀且可變的集合的抽象基類。

class collections.abc.Mapping?
class collections.abc.MutableMapping?

只讀且可變的映射 mappings 的抽象基類。

class collections.abc.MappingView?
class collections.abc.ItemsView?
class collections.abc.KeysView?
class collections.abc.ValuesView?

映射及其鍵和值的視圖 views 的抽象基類。

class collections.abc.Awaitable?

為可等待對(duì)象 awaitable 提供的類，可以被用于 await 表達(dá)式中。習(xí)慣上必須實(shí)現(xiàn) __await__() 方法。

協(xié)程 對(duì)象和 Coroutine ABC 的實(shí)例都是這個(gè) ABC 的實(shí)例。

備注

In CPython, generator-based coroutines (generators decorated with types.coroutine()) are awaitables, even though they do not have an __await__() method. Using isinstance(gencoro, Awaitable) for them will return False. Use inspect.isawaitable() to detect them.

3.5 新版功能.

class collections.abc.Coroutine?

用于協(xié)程兼容類的抽象基類。實(shí)現(xiàn)了如下定義在 協(xié)程對(duì)象: 里的方法： send()，throw() 和 close()。通常的實(shí)現(xiàn)里還需要實(shí)現(xiàn) __await__() 方法。所有的 Coroutine 實(shí)例都必須是 Awaitable 實(shí)例。參見 coroutine 的定義。

備注

In CPython, generator-based coroutines (generators decorated with types.coroutine()) are awaitables, even though they do not have an __await__() method. Using isinstance(gencoro, Coroutine) for them will return False. Use inspect.isawaitable() to detect them.

3.5 新版功能.

class collections.abc.AsyncIterable?

提供了 __aiter__ 方法的抽象基類。參見 asynchronous iterable 的定義。

3.5 新版功能.

class collections.abc.AsyncIterator?

提供了 __aiter__ 和 __anext__ 方法的抽象基類。參見 asynchronous iterator 的定義。

3.5 新版功能.

class collections.abc.AsyncGenerator?

為異步生成器類提供的抽象基類，這些類實(shí)現(xiàn)了定義在 PEP 525 和 PEP 492 里的協(xié)議。

3.6 新版功能.

Examples and Recipes?

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

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

有些抽象基類也可以用作混入類（mixin），這可以更容易地開發(fā)支持容器 API 的類。例如，要寫一個(gè)支持完整 Set API 的類，只需要提供下面這三個(gè)方法： __contains__(), __iter__() 和 __len__()。抽象基類會(huì)補(bǔ)充上其余的方法，比如 __and__() 和 isdisjoint():

class ListBasedSet(collections.abc.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

當(dāng)把 Set 和 MutableSet 用作混入類時(shí)需注意：

  1. 由于某些集合操作會(huì)創(chuàng)建新集合，默認(rèn)的混入方法需要一種從可迭代對(duì)象里創(chuàng)建新實(shí)例的方式。 假定類構(gòu)造器具有 ClassName(iterable) 形式的簽名。 這樣它將執(zhí)行一個(gè)名為 _from_iterable() 的內(nèi)部類方法，該方法會(huì)調(diào)用 cls(iterable) 來產(chǎn)生一個(gè)新集合。 如果 Set 混入類在具有不同構(gòu)造器簽名的類中被使用，你將需要通過類方法或常規(guī)方法來重載 _from_iterable()，以便基于可迭代對(duì)象參數(shù)來構(gòu)造新的實(shí)例。

  2. 重載比較符時(shí)時(shí)（想必是為了速度，因?yàn)槠湔Z義都是固定的），只需要重定義 __le__() 和 __ge__() 函數(shù)，然后其他的操作會(huì)自動(dòng)跟進(jìn)。

  3. 混入集合類 Set 提供了一個(gè) _hash() 方法為集合計(jì)算哈希值，然而， __hash__() 函數(shù)卻沒有被定義，因?yàn)椴⒉皇撬屑隙际强晒２⑶也豢勺兊?。為了使用混入類為集合添加哈希能力，可以同時(shí)繼承 Set() 和 Hashable() 類，然后定義 __hash__ = Set._hash。

參見