python的collection系列-有序字典（OrderedDict）

orderdDict是對字典類型的補充，他記住了字典元素添加的順序

注意：字典默認循環只輸出key

import collections
dic = collections.OrderedDict()
dic["k1"] = "v1"
dic["k2"] = "v2"
dic["k3"] = "v3"
print(dic)
#實現原理：相當於用列表（有序）來維護字典（無序）排序,以下僅供理解
# dic = {"k1":"v1","k2":"v2"}
# li = ["k1","k2"]
# for i in li:
#     print(dic.get(i))

執行結果：無論執行多少次結果一樣
OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3')])

def popitem(self, last=True): 2         '''od.popitem() -> (k, v), return and remove a (key, value) pair. 3  Pairs are returned in LIFO order if last is true or FIFO order if false.

#有序刪除和指定刪除
import collections
dic = collections.OrderedDict()
dic["k1"] = "v1"
dic["k2"] = "v2"
dic["k3"] = "v3"
print(dic)
dic.popitem()    #有序拿掉，每次拿掉最后一個，相當於內存的棧存放，后進先出原則，而pop()就是強制拿出指定的值
print(dic)

執行結果：
OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3')])
OrderedDict([('k1', 'v1'), ('k2', 'v2')])

def move_to_end(self, key, last=True): 2         '''Move an existing element to the end (or beginning if last==False). 3 
 Raises KeyError if the element does not exist. 5  When last=True, acts like a fast version of self[key]=self.pop(key).

#把指定鍵值移到最后
import collections
dic = collections.OrderedDict()
dic["k1"] = "v1"
dic["k2"] = "v2"
dic["k3"] = "v3"
print(dic)
dic.move_to_end("k1")    #把指定鍵值移到最后
print(dic)

#執行結果：
OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3')])
OrderedDict([('k2', 'v2'), ('k3', 'v3'), ('k1', 'v1')])

def setdefault(self, key, default=None): 2         'od.setdefault(k[,d]) -> od.get(k,d), also set od[k]=d if k not in od'
        if key in self: 4             return self[key] 5         self[key] = default 6         return default

#添加默認鍵
import collections
dic = collections.OrderedDict()
dic["k1"] = "v1"
dic["k2"] = "v2"
dic["k3"] = "v3"
print(dic)
dic.setdefault("k4","v4")    #默認鍵值為None,不過可以添加值
print(dic)

#執行結果：
OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3')])
OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3'), ('k4', 'v4')])