将字典添加到带有联合的 `set()` [英] Add a dictionary to a `set()` with union

问题描述

我刚刚遇到了一些我想我会问的有趣的事情.

I just ran across something interesting that I thought I'd ask about.

将字典添加到 set 中，我曾假设该字典将作为完整字典添加，但事实并非如此.仅添加键:

Adding a dictionary to a set, I had assumed that the dictionary would be added as a full dictionary, it isn't. Only the keys are added:

dicty = {"Key1": "Val1", "Key2": "Val2"}
setunion = set()
setunion.union(dicty)
=> set(['Key2', 'Key1'])

当您尝试使用 set.add() 添加它时，您会收到一个错误:

When you attempt to add it using set.add() you get an error:

setadd = set()
setadd.add(dicty)
Traceback (most recent call last):
  File "python", line 1, in <module>
TypeError: unhashable type: 'dict'

显然，这种行为与列表非常不同:

Obviously, this behaviour is very different from lists:

   listy = []
   listy.append(dicty)
   listy
=> [{'Key2': 'Val2', 'Key1': 'Val1'}]

在文档中 它说集合是 hashable 对象的无序集合，这暗示了上面的一些问题.

In the docs it says that sets are unordered collections of hashable objects, which is a hint to some of the issues above.

这是怎么回事?设置项必须是可散列的，很明显这与为什么我只使用 .union() 将键添加到集合中有关，但是为什么 .add() 出现错误?

What's going on here? Set items have to be hashable, so clearly that has to do with why I'm only adding the keys to the set with .union(), but why the error with .add()?

列表中集合的行为差异背后是否存在某些可用性原因?

Is there some usability reason behind the difference in behavior of sets from lists?

Python(或库)中是否有一种数据类型本质上类似于列表，但只保留唯一项?

Is there a datatype in Python (or a library) that essentially functions like a list, but only keeps unique items?

推荐答案

不，这是不可能的.哈希表(如dicts 和sets)进行查找的方式与数组(如lists)进行查找的方式从根本上是独一无二的.逻辑错误是，如果您有一个只保存重复项的数据类型，如果您将其中一个元素变异为非唯一元素，会发生什么情况?

No that's impossible by definition. The way hash tables (like dicts and sets) do lookups is fundamentally unique from the way arrays (like lists) do lookups. The logical error is that if you have a datatype that only saves duplicates, what happens if you mutate one of the elements to be non-unique?

a, b = [0], [0, 1]
s = SpecialSet(a, b)
a.append(1)  # NOW WHAT?!

如果你想给一个集合添加一个字典，你可以添加它的 dict.items 视图(它实际上只是一个元组列表)，但你必须先转换为元组.

If you want to add a dictionary to a set, you can add the dict.items view of it (which is really just a list of tuples), but you have to cast to tuple first.

a = {1:2, 3:4}
s = set()
s.add(tuple(a.items()))

然后你必须重新转换为 dict 一旦它离开集合以取回字典

Then you'd have to re-cast to dict that once it leaves the set to get a dictionary back

for tup in s:
    new_a = dict(tup)

在 PEP416 中提出了内置的 frozendict 类型但最终被拒绝.

A built-in frozendict type was proposed in PEP416 but ultimately rejected.

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