Python OrderedDict is a dict subclass that maintains the items insertion order. When we iterate over an OrderedDict, items are returned in the order they were inserted.

Python OrderedDict是一个dict子类，用于维护项目的插入顺序。 当我们遍历OrderedDict时，将按照插入顺序返回项目。

A regular dictionary doesn’t track the insertion order. So when iterating over it, items are returned in an arbitrary order. When we want to make sure that items are returned in the order they were inserted, we can use OrderedDict.

常规词典不会跟踪插入顺序。 因此，在对其进行迭代时，将以任意顺序返回项目。 当我们要确保按插入顺序返回项目时，可以使用OrderedDict。

Python OrderedDict (Python OrderedDict)

• OrderedDict is part of python collections module.OrderedDict是python集合模块的一部分。
• We can create an empty OrderedDict and add items to it. If we create an OrderedDict by passing a dict argument, then the ordering may be lost because dict doesn’t maintain the insertion order.我们可以创建一个空的OrderedDict并向其中添加项目。 如果我们通过传递dict参数来创建OrderedDict，则该顺序可能会丢失，因为dict无法维持插入顺序。
• If an item is overwritten in the OrderedDict, it’s position is maintained.如果某个项目在OrderedDict中被覆盖，则其位置将保持不变。
• If an item is deleted and added again, then it moves to the last.如果一个项目被删除并再次添加，则它将移至最后一个。
• OrderedDict popitem removes the items in FIFO order. It accepts a boolean argument last, if it’s set to True then items are returned in LIFO order.OrderedDict popitem按FIFO顺序删除项目。 它last接受一个布尔参数，如果将其设置为True则将按LIFO顺序返回项目。
• We can move an item to the beginning or end of the OrderedDict using move_to_end function. It accepts a boolean argument last, if it’s set to False then item is moved to the start of the ordered dict.我们可以使用move_to_end函数将项目移动到OrderedDict的开头或结尾。 它last接受一个布尔参数，如果将其设置为False则将项目移到有序dict的开头。
• From python 3.6 onwards, order is retained for keyword arguments passed to the OrderedDict constructor, refer PEP-468.从python 3.6开始，保留传递给OrderedDict构造函数的关键字参数的顺序，请参阅PEP-468 。
• We can use reversed() function with OrderedDict to iterate elements in the reverse order.我们可以使用带有OrderedDict的reversed()函数以相反的顺序迭代元素。
• Equality tests between OrderedDict objects are order-sensitive and are implemented as list(od1.items())==list(od2.items()).OrderedDict对象之间的相等性测试对顺序敏感，并实现为list(od1.items())==list(od2.items()) 。
• Equality tests between OrderedDict and other Mapping objects are order-insensitive like regular dictionaries. This allows OrderedDict objects to be substituted anywhere a regular dictionary is used.与常规字典一样，OrderedDict与其他Mapping对象之间的相等性测试对顺序不敏感。 这允许在使用常规词典的任何地方替换OrderedDict对象。

Python OrderedDict示例 (Python OrderedDict Examples)

Let’s look at some code examples of Python OrderedDict.

让我们看一下Python OrderedDict的一些代码示例。

创建OrderedDict对象 (Creating OrderedDict object)

from collections import OrderedDict# creating a simple dict
my_dict = {'kiwi': 4, 'apple': 5, 'cat': 3}# creating empty ordered dict
ordered_dict = OrderedDict()
print(ordered_dict)# creating ordered dict from dict
ordered_dict = OrderedDict(my_dict)
print(ordered_dict)

Output:

输出：

OrderedDict()
OrderedDict([('kiwi', 4), ('apple', 5), ('cat', 3)])

从OrderedDict添加，替换，删除项目 (Adding, Replacing, Removing items from OrderedDict)

# adding elements to dict
ordered_dict['dog'] = 3# replacing a dict key value
ordered_dict['kiwi'] = 10
print(ordered_dict)# removing and adding a value
ordered_dict.pop('kiwi')
print(ordered_dict)
ordered_dict['kiwi'] = 4
print(ordered_dict)

Output:

输出：

OrderedDict([('kiwi', 10), ('apple', 5), ('cat', 3), ('dog', 3)])
OrderedDict([('apple', 5), ('cat', 3), ('dog', 3)])
OrderedDict([('apple', 5), ('cat', 3), ('dog', 3), ('kiwi', 4)])

OrderedDict move_to_end示例 (OrderedDict move_to_end example)

# moving apple to end and dog to start
ordered_dict.move_to_end('apple')
ordered_dict.move_to_end('dog', False)
print(ordered_dict)

Output:

输出：

OrderedDict([('dog', 3), ('cat', 3), ('kiwi', 4), ('apple', 5)])

OrderedDict popitem示例 (OrderedDict popitem example)

# pop last item
item = ordered_dict.popitem(True)
print(item)
print(ordered_dict)

Output:

输出：

('apple', 5)
OrderedDict([('dog', 3), ('cat', 3), ('kiwi', 4)])

OrderedDict反向迭代 (OrderedDict Reverse Iteration)

# reversed iteration
for item in reversed(ordered_dict):print(item)

Output:

输出：

kiwi
cat
dog

OrderedDict相等性测试示例 (OrderedDict Equality Tests Example)

# equality tests
d1 = {'a': 'A', 'b': 'B'}
d2 = {'b': 'B', 'a': 'A'}od1 = OrderedDict({'a': 'A', 'b': 'B'})
od2 = OrderedDict({'b': 'B', 'a': 'A'})print(d1 == d2)
print(od1 == od2)
print(d1 == od1)

Output:

输出：

True
False
True

Reference: Python Docs

参考： Python文档

翻译自: https://www.journaldev.com/21807/python-ordereddict