"some_set in some_set_of_sets" contains a race condition that can lead to odd behavior.  To perform the test, the set_contains() function in setobject.c creates a temporary frozenset object (t), swaps the bodies of some_set and t, checks if t is in some_set_of_sets, then swaps the bodies back.

Unfortunately, comparisons or hash functions may release the GIL, so the swapped bodies may be exposed on a different thread, i.e., "some_set in some_set_of_sets" may cause "some_set" to be empty on some other thread.

The same race condition exists in set_discard() and set_remove().

Attached is a short script that demonstrates the problem and could be easily converted to a unit test.