>>> set() in set()
False  # should raise TypeError

>>> dict() in set()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'dict'

>>> set() in dict()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'set'

>>> dict() in dict()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'dict'

>>> frozenset({1,2}) in {frozenset({1,2}), 3}
True

>>> {1,2} in {frozenset({1,2}), 3}
True  # should raise TypeError

The set is converted to a frozenset before the comparison so 

>>> {1,2} in {frozenset({1,2}), 3}

is equivalent to 

>>> frozenset({1,2}) in {frozenset({1,2}), 3}


This was introduced on purpose 17 years ago in 19c2d778 so it's unlikely to change.

This is documented in https://docs.python.org/3.8/library/stdtypes.html#set-types-set-frozenset:


Instances of set are compared to instances of frozenset based on their members. For example, set('abc') == frozenset('abc') returns True and so does set('abc') in set([frozenset('abc')]).

thanks, it sounds reasonable.
is it possible to make dict give consistent result?
{1,2} in {frozenset({1,2}):3} 
# to return True

> is it possible to make dict give consistent result?

That is unlikely (there isn't much need, it would introduce a cross-type dependency, it would be awkward dicts which use keys in many more ways than sets do).

For the time being, a user should explicitly convert to a frozenset:

    frozenset(s) in d