python内置collections模块的使用

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虽然不是经常用，还是在这里记录一下吧

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一、collections模块说明

一个模块主要用来干嘛，有哪些类可以使用，看__init__.py就知道

1、查看collections模块的定义路径

查看属性：__file__

>>> import collections
>>> collections.__file__
'D:\\Anaconda_install\\lib\\collections\\__init__.py'

2、查看collections文档介绍信息

查看属性：__doc__

>>> import collections
>>> print(collections.__doc__)
This module implements specialized container datatypes providing
alternatives to Python's general purpose built-in containers, dict,
list, set, and tuple.

* namedtuple   factory function for creating tuple subclasses with named fields
* deque        list-like container with fast appends and pops on either end
* ChainMap     dict-like class for creating a single view of multiple mappings
* Counter      dict subclass for counting hashable objects
* OrderedDict  dict subclass that remembers the order entries were added
* defaultdict  dict subclass that calls a factory function to supply missing values
* UserDict     wrapper around dictionary objects for easier dict subclassing
* UserList     wrapper around list objects for easier list subclassing
* UserString   wrapper around string objects for easier string subclassing

中文介绍：
collections包含了一些特殊的容器，针对Python内置的容器，例如list、dict、set和tuple，提供了另一种选择；

• namedtuple，可以创建包含名称的tuple；

• deque，类似于list的容器，可以快速的在队列头部和尾部添加、删除元素；

• Counter，dict的子类，计算可hash的对象；

• OrderedDict，dict的子类，可以记住元素的添加顺序，是一个有序字典；

• defaultdict，dict的子类，带有默认值的字典；

3、查看collections的所有可用的类、方法和属性

使用函数：dir()

>>> import collections
>>> dir(collections)
['AsyncGenerator', 'AsyncIterable', 'AsyncIterator', 'Awaitable', 'ByteString', 
'Callable', 'ChainMap', 'Collection', 'Container', 'Coroutine', 'Counter', 
'Generator', 'Hashable', 'ItemsView', 'Iterable', 'Iterator', 'KeysView', 
'Mapping', 'MappingView', 'MutableMapping', 'MutableSequence', 'MutableSet', 
'OrderedDict', 'Reversible', 'Sequence', 'Set', 'Sized', 'UserDict', 'UserList', 
'UserString', 'ValuesView', '_Link', '_OrderedDictItemsView',
'_OrderedDictKeysView', '_OrderedDictValuesView', '__all__', '__builtins__', 
'__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', 
'__path__', '__spec__', '_chain', '_class_template', '_collections_abc',
'_count_elements', '_eq', '_field_template', '_heapq', '_iskeyword', 
'_itemgetter', '_proxy', '_recursive_repr', '_repeat', '_repr_template',
'_starmap', '_sys', 'abc', 'defaultdict', 'deque', 'namedtuple']

二、collections模块的具体使用

1、namedtuple（)函数的使用

namedtuple生成可以使用名字来访问元素内容的tuple
1、查看一下namedtuple有哪些属性

>>> dir(namedtuple)
['__annotations__', '__call__', '__class__', '__closure__', '__code__', '__defaults__', '__delattr__', 
'__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__get__', '__getattribute__',
 '__globals__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__kwdefaults__', '__le__', 
 '__lt__', '__module__', '__name__', '__ne__', '__new__', '__qualname__', '__reduce__', '__reduce_ex__', 
 '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

2、namedtuple的官方文档

>>> print(namedtuple.__doc__)
Returns a new subclass of tuple with named fields.

    >>> Point = namedtuple('Point', ['x', 'y'])
    >>> Point.__doc__                   # docstring for the new class
    'Point(x, y)'
    >>> p = Point(11, y=22)             # instantiate with positional args or keywords
    >>> p[0] + p[1]                     # indexable like a plain tuple
    33
    >>> x, y = p                        # unpack like a regular tuple
    >>> x, y
    (11, 22)
    >>> p.x + p.y                       # fields also accessible by name
    33
    >>> d = p._asdict()                 # convert to a dictionary
    >>> d['x']
    11
    >>> Point(**d)                      # convert from a dictionary
    Point(x=11, y=22)
    >>> p._replace(x=100)               # _replace() is like str.replace() but targets named fields

3、namedtuple的具体使用举例
定义：namedtuple('名称', [ 属性list]):

下面定义一个圆，可以通过元组对应的名字来访问对应的值，下面访问圆的圆心坐标和半径

>>> from collections import namedtuple
>>> circle = namedtuple("Circle", ["x", "y", "r"])
>>> circle
<class '__main__.Circle'>
>>> circle1 = circle(2,4,6)
>>> circle1
Circle(x=2, y=4, r=6)
>>> circle1.x
2
>>> circle1.y
4
>>> circle1.r
6

2、defaultdict类的具体使用

defaultdict带有默认值的字典
1、查看defaultdict类的方法和属性

>>> dir(defaultdict)
['__class__', '__contains__', '__copy__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', 
'__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', 
'__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', '__missing__', '__ne__', '__new__', 
'__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setitem__', '__sizeof__', '__str__',
 '__subclasshook__', 'clear', 'copy', 'default_factory', 'fromkeys', 'get', 'items', 'keys', 'pop', 
 'popitem', 'setdefault', 'update', 'values']

因为defaultdict是继承了dict类的，所以dict的方法属性defaultdict都可以用，下面列出dict的方法和属性，你和上面的对比一下

>>> dir(dict)
['__class__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__',
 '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__',
  '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', 
  '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setitem__', '__sizeof__', 
  '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'items', 'keys', 'pop', 
  'popitem', 'setdefault', 'update', 'values']

2、defaultdict的官方文档

>>> print(defaultdict.__doc__)
defaultdict(default_factory[, ...]) --> dict with default factory

The default factory is called without arguments to produce
a new value when a key is not present, in __getitem__ only.
A defaultdict compares equal to a dict with the same items.
All remaining arguments are treated the same as if they were
passed to the dict constructor, including keyword arguments.

3、defaultdict的具体使用举例

3、Counter类的使用

1、counter类的使用
Counter类的目的是用来跟踪值出现的次数。它是一个无序的容器类型，以字典的键值对形式存储，其中元素作为key，其计数作为value。计数值可以是任意的Interger（包括0和负数）。Counter类和其他语言的bags或multisets很相似。

>>> from collections import Counter
>>> c = Counter('abcdeabcdabcaba')
>>> c
Counter({'a': 5, 'b': 4, 'c': 3, 'd': 2, 'e': 1})

2、Counter对象创建的四种方法

>>> c = Counter()  # 创建一个空的Counter类
>>> c = Counter('gallahad')  # 从一个可iterable对象（list、tuple、dict、字符串等）创建
>>> c = Counter({'a': 4, 'b': 2})  # 从一个字典对象创建
>>> c = Counter(a=4, b=2)  # 从一组键值对创建

正常访问字典，如果键不存在会返回KeyError, 但是Counter不会，它返回的是0，因为它返回的是个数

>>> from collections import Counter
>>> c = Counter('abcdeabcdabcaba')
>>> c
Counter({'a': 5, 'b': 4, 'c': 3, 'd': 2, 'e': 1})
>>> b = dict(c)
>>> b
{'a': 5, 'b': 4, 'c': 3, 'd': 2, 'e': 1}
>>> b["name"]
Traceback (most recent call last):
  File "", line 1, in <module>
KeyError: 'name'
>>> c['a']
5
>>> c['name']
0

三、collections官方给出的具体定义，方便参考

1、namedtuple函数

namedtuple生成可以使用名字来访问元素内容的tuple

def namedtuple(typename, field_names, *, verbose=False, rename=False, module=None):
    """Returns a new subclass of tuple with named fields.

    >>> Point = namedtuple('Point', ['x', 'y'])
    >>> Point.__doc__                   # docstring for the new class
    'Point(x, y)'
    >>> p = Point(11, y=22)             # instantiate with positional args or keywords
    >>> p[0] + p[1]                     # indexable like a plain tuple
    33
    >>> x, y = p                        # unpack like a regular tuple
    >>> x, y
    (11, 22)
    >>> p.x + p.y                       # fields also accessible by name
    33
    >>> d = p._asdict()                 # convert to a dictionary
    >>> d['x']
    11
    >>> Point(**d)                      # convert from a dictionary
    Point(x=11, y=22)
    >>> p._replace(x=100)               # _replace() is like str.replace() but targets named fields
    Point(x=100, y=22)

    """

    # Validate the field names.  At the user's option, either generate an error
    # message or automatically replace the field name with a valid name.
    if isinstance(field_names, str):
        field_names = field_names.replace(',', ' ').split()
    field_names = list(map(str, field_names))
    typename = str(typename)
    if rename:
        seen = set()
        for index, name in enumerate(field_names):
            if (not name.isidentifier()
                or _iskeyword(name)
                or name.startswith('_')
                or name in seen):
                field_names[index] = '_%d' % index
            seen.add(name)
    for name in [typename] + field_names:
        if type(name) is not str:
            raise TypeError('Type names and field names must be strings')
        if not name.isidentifier():
            raise ValueError('Type names and field names must be valid '
                             'identifiers: %r' % name)
        if _iskeyword(name):
            raise ValueError('Type names and field names cannot be a '
                             'keyword: %r' % name)
    seen = set()
    for name in field_names:
        if name.startswith('_') and not rename:
            raise ValueError('Field names cannot start with an underscore: '
                             '%r' % name)
        if name in seen:
            raise ValueError('Encountered duplicate field name: %r' % name)
        seen.add(name)

    # Fill-in the class template
    class_definition = _class_template.format(
        typename = typename,
        field_names = tuple(field_names),
        num_fields = len(field_names),
        arg_list = repr(tuple(field_names)).replace("'", "")[1:-1],
        repr_fmt = ', '.join(_repr_template.format(name=name)
                             for name in field_names),
        field_defs = '\n'.join(_field_template.format(index=index, name=name)
                               for index, name in enumerate(field_names))
    )

    # Execute the template string in a temporary namespace and support
    # tracing utilities by setting a value for frame.f_globals['__name__']
    namespace = dict(__name__='namedtuple_%s' % typename)
    exec(class_definition, namespace)
    result = namespace[typename]
    result._source = class_definition
    if verbose:
        print(result._source)

    # For pickling to work, the __module__ variable needs to be set to the frame
    # where the named tuple is created.  Bypass this step in environments where
    # sys._getframe is not defined (Jython for example) or sys._getframe is not
    # defined for arguments greater than 0 (IronPython), or where the user has
    # specified a particular module.
    if module is None:
        try:
            module = _sys._getframe(1).f_globals.get('__name__', '__main__')
        except (AttributeError, ValueError):
            pass
    if module is not None:
        result.__module__ = module

    return result

2、defaultdict类

defaultdict: 带有默认值的字典，从面的定义可以看出，defaultdict是dict的子类，因此会继承dict的所有方法和属性

class defaultdict(dict):
    """
    defaultdict(default_factory[, ...]) --> dict with default factory
    
    The default factory is called without arguments to produce
    a new value when a key is not present, in __getitem__ only.
    A defaultdict compares equal to a dict with the same items.
    All remaining arguments are treated the same as if they were
    passed to the dict constructor, including keyword arguments.
    """
    def copy(self): # real signature unknown; restored from __doc__
        """ D.copy() -> a shallow copy of D. """
        pass

    def __copy__(self, *args, **kwargs): # real signature unknown
        """ D.copy() -> a shallow copy of D. """
        pass

    def __getattribute__(self, *args, **kwargs): # real signature unknown
        """ Return getattr(self, name). """
        pass

    def __init__(self, default_factory=None, **kwargs): # known case of _collections.defaultdict.__init__
        """
        defaultdict(default_factory[, ...]) --> dict with default factory
        
        The default factory is called without arguments to produce
        a new value when a key is not present, in __getitem__ only.
        A defaultdict compares equal to a dict with the same items.
        All remaining arguments are treated the same as if they were
        passed to the dict constructor, including keyword arguments.
        
        # (copied from class doc)
        """
        pass

    def __missing__(self, key): # real signature unknown; restored from __doc__
        """
        __missing__(key) # Called by __getitem__ for missing key; pseudo-code:
          if self.default_factory is None: raise KeyError((key,))
          self[key] = value = self.default_factory()
          return value
        """
        pass

    def __reduce__(self, *args, **kwargs): # real signature unknown
        """ Return state information for pickling. """
        pass

    def __repr__(self, *args, **kwargs): # real signature unknown
        """ Return repr(self). """
        pass

    default_factory = property(lambda self: object(), lambda self, v: None, lambda self: None)  # default
    """Factory for default value called by __missing__()."""

参考：
1、http://www.cnblogs.com/zhbzz2007/p/5986369.html
2、https://www.jianshu.com/p/efb85d856cc9
3、https://blog.csdn.net/abc_12366/article/details/80447228
4、太长
5、https://www.zlovezl.cn/articles/collections-in-python/

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