Python中的有序字典(OrderedDict)

1 年 ago

雅, 悟

3 minutes

Python的OrderedDict是一个字典子类，它会保持项目插入的顺序。当我们遍历OrderedDict时，项目会按照它们被插入的顺序返回。普通字典不会追踪插入顺序。因此，在对其进行迭代时，项目会按照任意的顺序返回。当我们想要确保项目按照插入的顺序返回时，我们可以使用OrderedDict。

Python有序字典

OrderedDict is part of python collections module.
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.
If an item is overwritten in the OrderedDict, it’s position is maintained.
If an item is deleted and added again, then it moves to the last.
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.
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.
From python 3.6 onwards, order is retained for keyword arguments passed to the OrderedDict constructor, refer PEP-468.
We can use reversed() function with OrderedDict to iterate elements in the reverse order.
Equality tests between OrderedDict objects are order-sensitive and are implemented as 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.

Python有序字典示例

让我们看一些Python OrderedDict的代码示例。

创建有序字典对象

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)

结果：

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

在有序字典(OrderedDict)中添加、替换、移除项目。

# 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)

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)])

有序字典 move_to_end 示例

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

输出：

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

有序字典popitem示例

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

输出：产出/产量

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

有序字典的逆向迭代

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

产出：输出

kiwi
cat
dog

有序字典的相等性测试示例

# 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)

输出：

True
False
True

你可以从我们的GitHub代码库中下载完整的示例代码。

参考资料：Python文档