Python OrderedDict 不保持元素顺序 [英] Python OrderedDict not keeping element order

问题描述

我正在尝试创建一个 OrderedDict 对象，但我刚创建它，元素就变得混乱了.

这就是我所做的:

from collections import OrderedDictod = OrderedDict({(0,0):[2],(0,1):[1,9],(0,2):[1,5,9]})

元素没有按照我指定的顺序保持

odOrderedDict([((0, 1), [1, 9]), ((0, 0), [2]), ((0, 2), [1, 5, 9])])

docs.python.org 没有示例，我无法弄清楚为什么订单会变得混乱.任何帮助是极大的赞赏.

解决方案

您的问题是您正在构建一个 dict 以将初始数据提供给 OrderedDict - 这dict 不存储任何订单，因此订单在到达OrderedDict之前就丢失了.

解决方案是从有序数据类型构建 - 最简单的是 tuple 的 list:

<预><代码>>>>从集合导入 OrderedDict>>>od = OrderedDict([((0, 0), [2]), ((0, 1), [1, 9]), ((0, 2), [1, 5, 9])])>>>外径OrderedDict([((0, 0), [2]), ((0, 1), [1, 9]), ((0, 2), [1, 5, 9])])

值得注意的是，这就是 OrderedDict 使用它为字符串表示所做的语法的原因 - 字符串表示应该尝试成为有效的 Python 代码以在可能的情况下重现对象，这就是为什么输出使用元组列表而不是字典.

从 Python 3.6 开始, kwargs 是有序的，因此您可以使用关键字参数代替，前提是您使用的是最新的 Python 版本.

从 3.7 开始，对于 dict<也是如此/code>s(它在 3.6 中用于 CPython，但语言规范没有指定它，因此为了兼容性仍然需要使用 OrderedDict).这意味着如果您可以假设 3.7+ 环境，您通常可以完全删除 OrderedDict，或者如果您需要特定功能(例如:订购到平等的问题).

I'm trying to create an OrderedDict object but no sooner do I create it, than the elements are all jumbled.

This is what I do:

from collections import OrderedDict
od = OrderedDict({(0,0):[2],(0,1):[1,9],(0,2):[1,5,9]})

The elements don't stay in the order I assign

od
OrderedDict([((0, 1), [1, 9]), ((0, 0), [2]), ((0, 2), [1, 5, 9])])

docs.python.org doesn't have an example and I can't figure out why the order is getting jumbled. Any help is greatly appreciated.

解决方案

Your problem is that you are constructing a dict to give the initial data to the OrderedDict - this dict doesn't store any order, so the order is lost before it gets to the OrderedDict.

The solution is to build from an ordered data type - the easiest being a list of tuples:

>>> from collections import OrderedDict
>>> od = OrderedDict([((0, 0), [2]), ((0, 1), [1, 9]), ((0, 2), [1, 5, 9])])
>>> od
OrderedDict([((0, 0), [2]), ((0, 1), [1, 9]), ((0, 2), [1, 5, 9])])

It's worth noting that this is why OrderedDict uses the syntax it does for it's string representation - string representations should try to be valid Python code to reproduce the object where possible, and that's why the output uses a list of tuples instead of a dict.

Edit: As of Python 3.6, kwargs is ordered, so you can use keyword arguments instead, provided you are on an up-to-date Python version.

As of 3.7, this is also true for dicts (it was for CPython in 3.6, but the language spec didn't specify it, so using OrderedDict was still required for compatibility). This means if you can assume a 3.7+ environment, you can often drop OrderedDict altogether, or construct one from a regular dict if you need a specific feature (e.g: order to matter for equality).

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