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

 1 import collections
 2 dic = collections.OrderedDict()
 3 dic["k1"] = "v1"
 4 dic["k2"] = "v2"
 5 dic["k3"] = "v3"
 6 print(dic)
 7 #實現原理：至關於用列表（有序）來維護字典（無序）排序,如下僅供理解
 8 # dic = {"k1":"v1","k2":"v2"}
 9 # li = ["k1","k2"]
10 # for i in li:
11 #     print(dic.get(i))
12 
13 執行結果：不管執行多少次結果同樣
14 OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3')])

1 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.

 1 #有序刪除和指定刪除
 2 import collections
 3 dic = collections.OrderedDict()
 4 dic["k1"] = "v1"
 5 dic["k2"] = "v2"
 6 dic["k3"] = "v3"
 7 print(dic)
 8 dic.popitem()    #有序拿掉，每次拿掉最後一個，至關於內存的棧存放，後進先出原則，而pop()就是強制拿出指定的值
 9 print(dic)
10 
11 執行結果：
12 OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3')])
13 OrderedDict([('k1', 'v1'), ('k2', 'v2')])

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

 1 #把指定鍵值移到最後
 2 import collections
 3 dic = collections.OrderedDict()
 4 dic["k1"] = "v1"
 5 dic["k2"] = "v2"
 6 dic["k3"] = "v3"
 7 print(dic)
 8 dic.move_to_end("k1")    #把指定鍵值移到最後
 9 print(dic)
10 
11 #執行結果：
12 OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3')])
13 OrderedDict([('k2', 'v2'), ('k3', 'v3'), ('k1', 'v1')])

1 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'
3         if key in self: 4             return self[key] 5         self[key] = default 6         return default

 1 #添加默認鍵
 2 import collections
 3 dic = collections.OrderedDict()
 4 dic["k1"] = "v1"
 5 dic["k2"] = "v2"
 6 dic["k3"] = "v3"
 7 print(dic)
 8 dic.setdefault("k4","v4")    #默認鍵值爲None,不過能夠添加值
 9 print(dic)
10 
11 #執行結果：
12 OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3')])
13 OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3'), ('k4', 'v4')])