Sorry there was a typo in the first example block:

It should be

>>> {}.keys() & []
set()
>>> set() & []
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for &: 'set' and 'list'

>>> set() & [] does **not** return set() it raises a TypeError