The documentation for set operations says:
Note, the non-operator versions of union(), intersection(), difference(), symmetric_difference(), issubset(), and issuperset() methods will accept any iterable as an argument. In contrast, their operator based counterparts require their arguments to be sets. This precludes error-prone constructions like
set('abc') & 'cbs'
in favor of the more readableset('abc').intersection('cbs')
.
Testing this with the following experiment:
# 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
So, here is the paradox:
set
operator&
works withdict_keys
objects;- The documentation says it should only work with sets;
dict_keys
objects are not sets.
Why does set operator &
work with dict_keys objects? Are there other types that it works with? How can I find a list of these types?
CodePudding user response:
this is not a complete answer, but dict_keys
are instances of collections.abc.Set
:
from collections.abc import Set
k = dict(zip('ac', 'ac')).keys()
print(isinstance(k, Set)) # -> True