Python3.x 集合操作：应该只适用于集合，但适用于 dict_keys

documentation for set operations人说:

注意，union()、intersection()的非运算符版本，

通过以下实验来测试这一点:

# Python 3.10.2 (main, Jan 15 2022, 19:56:27) [GCC 11.1.0] on linux

>>> set('ab') & set('ac')
{'a'}
# works, as expected

>>> set('ab') & 'ac'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for &: 'set' and 'str'
# doesn't work, as expected

>>> set('ab') & list('ac')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for &: 'set' and 'list'
# doesn't work, as expected

>>> set('ab') & iter('ac')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for &: 'set' and 'str_iterator'
# doesn't work, as expected

>>> set('ab') & dict(zip('ac', 'ac')).keys()
{'a'}
# works??

>>> type({}.keys())
<class 'dict_keys'>

>>> isinstance({}.keys(), (set, frozenset))
False

所以，这里有一个悖论:

• set操作员&处理dict_keys个对象；
• 文件中说，它应该只适用于集合；
• dict_keys个对象不是集合.

为什么set运算符&与dict_keys对象一起工作？还有其他类型的可以使用吗？如何找到这些类型的列表？