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<title>5.15.3 Protocol for automatic conversion to immutable
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<H2><A NAME=
"SECTION0071530000000000000000"></A><A NAME=
"immutable-transforms"></A>
5.15.3 Protocol for automatic conversion to immutable
Sets can only contain immutable elements. For convenience, mutable
<tt class=
"class">Set
</tt> objects are automatically copied to an
<tt class=
"class">ImmutableSet
</tt>
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
<tt class=
"method">__as_immutable__()
</tt> method which returns an immutable equivalent.
Since
<tt class=
"class">Set
</tt> objects have a
<tt class=
"method">__as_immutable__()
</tt> method
returning an instance of
<tt class=
"class">ImmutableSet
</tt>, it is possible to
A similar mechanism is needed by the
<tt class=
"method">__contains__()
</tt> and
<tt class=
"method">remove()
</tt> 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
<tt class=
"method">__as_temporarily_immutable__()
</tt> method
which returns the element wrapped by a class that provides temporary
methods for
<tt class=
"method">__hash__()
</tt>,
<tt class=
"method">__eq__()
</tt>, and
<tt class=
"method">__ne__()
</tt>.
The alternate mechanism spares the need to build a separate copy of
the original mutable object.
<tt class=
"class">Set
</tt> objects implement the
<tt class=
"method">__as_temporarily_immutable__()
</tt>
method which returns the
<tt class=
"class">Set
</tt> object wrapped by a new class
<tt class=
"class">_TemporarilyImmutableSet
</tt>.
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
<tt class=
"class">_TemporarilyImmutableSet
</tt>. In other words, sets of mutable sets
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<span class=
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2005.
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