python - dict和set之间的区别(python)

Question

所以，我知道这个，

a = {}  # dict

构造一个空字典。现在，我也捡到了这个，

b = {1, 2, 3}  # set

创建一个集合。这很容易验证，因为，

>>>print(type(a))
<class 'dict'>

>>>print(type(b))
<class 'set'>

虽然我理解它的作用，但我看不出为什么我们对空字典使用“集合表示法”。我试图在 set 中找到有关其背后逻辑的更多信息。和 dict手册的部分，但遗憾的是，我没有从中得到任何东西。

谁能向我解释为什么我们会这样做？是因为历史原因，还是我遗漏了一些明显的东西？

Answer 1

Python 2 中没有 set literals，历史上花括号仅用于字典。集合可以从列表(或任何可迭代对象)中产生:

set([1, 2, 3])
set([i for i in range(1, 3)])

Python 3 引入了集合字面量和推导式(参见 PEP-3100)，这使我们能够避免中间列表:

{1, 2, 3}
{i for i in range(1, 3)}

然而，由于向后兼容，空集形式被保留给字典使用。来自 [Python-3000] sets in P3K? 的引用资料状态:

I'm sure we can work something out --- I agree, {} for empty set and {:} for empty dict would be ideal, were it not for backward compatibility. I liked the "special empty object" idea when I first wrote the PEP (i.e., have {} be something that could turn into either a set or dict), but one of the instructors here convinced me that it would just lead to confusion in newcomers' minds (as well as being a pain to implement).

following message更好地描述这些规则:

I think Guido had the best solution. Use set() for empty sets, use {} for empty dicts, use {genexp} for set comprehensions/displays, use {1,2,3} for explicit set literals, and use {k1:v1, k2:v2} for dict literals. We can always add {/} later if demand exceeds distaste.