5.13.3 Protocol for automatic conversion to immutable
Sets can only contain immutable elements. For convenience, mutable
Set objects are automatically copied to an ImmutableSet
before being added as a set element.
The mechanism is to always add a hashable element, or if it is not
hashable, the element is checked to see if it has an
__as_immutable__() method which returns an immutable equivalent.
Since Set objects have a __as_immutable__() method
returning an instance of ImmutableSet, it is possible to
construct sets of sets.
A similar mechanism is needed by the __contains__() and
remove() methods which need to hash an element to check
for membership in a set. Those methods check an element for hashability
and, if not, check for a __as_temporarily_immutable__() method
which returns the element wrapped by a class that provides temporary
methods for __hash__(), __eq__(), and __ne__().
The alternate mechanism spares the need to build a separate copy of
the original mutable object.
Set objects implement the __as_temporarily_immutable__()
method which returns the Set object wrapped by a new class
_TemporarilyImmutableSet.
The two mechanisms for adding hashability are normally invisible to the
user; however, a conflict can arise in a multi-threaded environment
where one thread is updating a set while another has temporarily wrapped it
in _TemporarilyImmutableSet. In other words, sets of mutable sets
are not thread-safe.
