为什么集合运算符使用 dict_key 视图对象而不是等效的集合方法？

Question

编辑：可能重复。 只有在发布这个问题并查看 "related questions" 之后我才能找到 Why are set methods like .intersection() not supported on set-like objects?，这个问题可能与重复。

我正在尝试查看字典中的键是否是集合的子集，并发现 dict_keys 视图对象和集合 methods/operators.

的一些令人困惑的行为

Difference between a set and a view指出：

Only the dict.keys() dictionary view is always a set (insofar that it behaves like a set, but one with a live view of the dictionary)

https://docs.python.org/3/library/stdtypes.html#dictionary-view-objects 文档指出

Keys views are set-like since their entries are unique and hashable. ... For set-like views, all of the operations defined for the abstract base class collections.abc.Set are available (for example, ==, <, or ^).

和 https://docs.python.org/2/library/stdtypes.html#set 集类型文档暗示运算符（<=、| 等）等同于方法 .issubset()、union() 等

但以下代码并未反映这些断言：

> dic = {'first': 'a', 'second': 'b'}

> s = {'first', 'second', 'third'}

> dic.keys() <= s

True

> dic.keys().issubset(s)


---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-354-fbf8f9744a80> in <module>
----> 1 dic.keys().issubset(s)

AttributeError: 'dict_keys' object has no attribute 'issubset'

然而

s.issuperset(dic.keys()) return True。

当所有迹象都表明 .issubset() 方法可以工作时，为什么会出现不同的行为？

Answer 1

作为对 Why are set methods like .intersection() not supported on set-like objects? 的回答指出，

dict views only guarantee the API of collections.abc.Set, not to be equivalent to set itself (...) these set-like objects are not claimed to match set, or even frozenset, just collections.abc.Set (and) collections.abc.Set doesn't require any of the named methods aside from isdisjoint (which has no operator equivalent).

为什么集合运算符使用 dict_key 视图对象而不是等效的集合方法？

Why do set operators work with dict_key view objects but not the equivalent set methods?

python

operators

set

hashable

dictview