python的collection系列-有序字典（OrderedDict）

orderdDict是对字典类型的补充，他记住了字典元素添加的顺序

注意：字典默认循环只输出key

 1 import collections
 2 dic = collections.OrderedDict()
 3 dic["k1"] = "v1"
 4 dic["k2"] = "v2"
 5 dic["k3"] = "v3"
 6 print(dic)
 7 #实现原理：相当于用列表（有序）来维护字典（无序）排序,以下仅供理解
 8 # dic = {"k1":"v1","k2":"v2"}
 9 # li = ["k1","k2"]
10 # for i in li:
11 #     print(dic.get(i))
12 
13 执行结果：无论执行多少次结果一样
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 #有序删除和指定删除
 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，每次拿掉最后一个，相当于内存的栈存放，后进先出原则，而pop()就是强制拿出指定的值
 9 print(dic)
10 
11 执行结果：
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 #把指定键值移到最后
 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")    #把指定键值移到最后
 9 print(dic)
10 
11 #执行结果：
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 #添加默认键
 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")    #默认键值为None,不过可以添加值
 9 print(dic)
10 
11 #执行结果：
12 OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3')])
13 OrderedDict([('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3'), ('k4', 'v4')])