python 内置模块--collections

1.计数器（counter）

 Counter是对字典的补充，用于追踪值出现的次数。

 Counter具有字典的全部属性和自己的属性。

>>>import collections

obj = collections.Counter('asasasasfageegadgsdga')

print(obj)

ret = obj.most_common(3)  #取出计数列的前3项
print(ret)

for i in obj.elements(): #elements用来取出计数器中的所有元素。
    print(i)

for k,v in obj.items():  #用items取出计数器中的keys和values
    print(k,v)

>>>obj = collections.Counter(['11', '22', '33', '22', '11', '22'])

print(obj)
obj.update(['11', '11', 'alex']) #更新数据
print(obj)

obj.subtract(['11'])  #从计数器中减去相应的元素

obj.__delitem__(['11'])  #删除一个元素

2.有序字典：

dic = collections.OrderedDict()

dic['K1'] = 'V1'
dic['K2'] = 'V2'
dic['K3'] = 'V3'
dic['K4'] = 'V4'

dic.update({'K1':'V232', 'K5':'V5'}) #数据存在的话就更新，不存在的话就添加

dic.clear()    #清空

dic.copy()     #复制

class OrderedDict(dict):
    """ Dictionary that remembers insertion order """
    def clear(self): # real signature unknown; restored from __doc__
        """ od.clear() -> None.  Remove all items from od. """
        pass

    def copy(self): # real signature unknown; restored from __doc__
        """ od.copy() -> a shallow copy of od """
        pass

    def fromkeys(cls, S, v=None): # real signature unknown; restored from __doc__
        """
        OD.fromkeys(S[, v]) -> New ordered dictionary with keys from S.
                If not specified, the value defaults to None.
        """
        pass

    def items(self, *args, **kwargs): # real signature unknown
        pass

    def keys(self, *args, **kwargs): # real signature unknown
        pass

    def move_to_end(self, *args, **kwargs): # real signature unknown
        """
        Move an existing element to the end (or beginning if last==False).

                Raises KeyError if the element does not exist.
                When last=True, acts like a fast version of self[key]=self.pop(key).
        """
        pass

    def pop(self, k, d=None): # real signature unknown; restored from __doc__
        """
        od.pop(k[,d]) -> v, remove specified key and return the corresponding
                value.  If key is not found, d is returned if given, otherwise KeyError
                is raised.
        """
        pass

    def popitem(self): # real signature unknown; restored from __doc__
        """
        od.popitem() -> (k, v), return and remove a (key, value) pair.
                Pairs are returned in LIFO order if last is true or FIFO order if false.
        """
        pass

    def setdefault(self, k, d=None): # real signature unknown; restored from __doc__
        """ od.setdefault(k[,d]) -> od.get(k,d), also set od[k]=d if k not in od """
        pass

    def update(self, *args, **kwargs): # real signature unknown
        pass

    def values(self, *args, **kwargs): # real signature unknown
        pass

3.默认字典（defaultdict）：

dic = collections.defaultdict(list)

在默认字典中，可以指定字典的values值的类型

eg：

import collections
dic = collections.defaultdict(list)
num = [11, 22, 33, 44, 55, 66, 77, 7, 88, 99]
for i in num:
    if i>44:
        dic['k1'].append(i)
    else:
        dic['k2'].append(i)
print(dic)