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5.7.3 Protocol for automatic conversion to immutable

5.7.3 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.

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