Python中的常规字典与有序字典

Python中的字典是数据值的无序集合，用于像地图一样存储数据值，与其他仅将单个值作为元素保存的数据类型不同，字典包含键：值对。字典中提供了键值，使其更加优化。常规字典类型不跟踪（键，值）对的插入顺序，因此根据它们在哈希表中的存储方式对键进行迭代，而哈希表又基于随机值，以减少冲突。
与此相反， Python提供了 OrderedDict 类型，它记住字典中 (key, value) 对的插入顺序，从而保留了顺序。 OrderedDict 比Python中的常规字典消耗更多内存，因为底层的双链表实现保留了顺序。

例子：

Python

# A Python program to demonstrate
# the difference between regular
# and ordered dictionary.
 
 
import collections
  
 
# Creating a regular dictionary
print('Regular dictionary:')
d = {chr(k):k for k in range(ord('a'), ord('g'))}
  
for k, v in d.items():
    print(k, v)
  
# Creating an Ordered dictionary
print('\nOrderedDict:')
d = collections.OrderedDict()
[d.setdefault(chr(k), k) for k in range(ord('a'), ord('g'))]
  
for k, v in d.items():
    print(k, v)

输出 ：

Regular dictionary:
('a', 97)
('c', 99)
('b', 98)
('e', 101)
('d', 100)
('f', 102)

OrderedDict:
('a', 97)
('b', 98)
('c', 99)
('d', 100)
('e', 101)
('f', 102)

注意：从Python 3.7 开始，保证Python字典的插入顺序。

删除和重新插入：
删除并重新插入相同的密钥会将其推到后面，因为 OrderedDict 但会保持插入顺序。

例子：

Python

# A Python program to demonstrate
# working of deletion and re-insertion in
# regular and OrderedDict
 
 
from collections import OrderedDict
   
print("Before deleting:\n")
 
d = {}
print("Regular dictionary:")
d['a'] = 1
d['b'] = 2
d['c'] = 3
d['d'] = 4
for key, value in d.items():
    print(key, value)
     
od = OrderedDict()
print("\nOrdered dictionary:")
od['a'] = 1
od['b'] = 2
od['c'] = 3
od['d'] = 4
for key, value in od.items():
    print(key, value)
 
   
print("\nAfter deleting:\n")
 
 
print("Regular dictionary:")
d.pop('c')
for key, value in d.items():
    print(key, value)
     
print("\nOrdered dictionary:")
od.pop('c')
for key, value in od.items():
    print(key, value)
   
   
print("\nAfter re-inserting:\n")
 
 
print("Regular dictionary:")
d['c'] = 3
for key, value in d.items():
    print(key, value)
     
print("\nOrdered dictionary:")
od['c'] = 3
for key, value in od.items():
    print(key, value)

输出：

Before deleting:

Regular dictionary:
('a', 1)
('c', 3)
('b', 2)
('d', 4)

Ordered dictionary:
('a', 1)
('b', 2)
('c', 3)
('d', 4)

After deleting:

Regular dictionary:
('a', 1)
('b', 2)
('d', 4)

Ordered dictionary:
('a', 1)
('b', 2)
('d', 4)

After re-inserting:

Regular dictionary:
('a', 1)
('c', 3)
('b', 2)
('d', 4)

Ordered dictionary:
('a', 1)
('b', 2)
('d', 4)
('c', 3)