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55 | <H2><A NAME="SECTION0071530000000000000000"></A><A NAME="immutable-transforms"></A> | |
56 | <BR> | |
57 | 5.15.3 Protocol for automatic conversion to immutable | |
58 | ||
59 | </H2> | |
60 | ||
61 | <P> | |
62 | Sets can only contain immutable elements. For convenience, mutable | |
63 | <tt class="class">Set</tt> objects are automatically copied to an <tt class="class">ImmutableSet</tt> | |
64 | before being added as a set element. | |
65 | ||
66 | <P> | |
67 | The mechanism is to always add a hashable element, or if it is not | |
68 | hashable, the element is checked to see if it has an | |
69 | <tt class="method">__as_immutable__()</tt> method which returns an immutable equivalent. | |
70 | ||
71 | <P> | |
72 | Since <tt class="class">Set</tt> objects have a <tt class="method">__as_immutable__()</tt> method | |
73 | returning an instance of <tt class="class">ImmutableSet</tt>, it is possible to | |
74 | construct sets of sets. | |
75 | ||
76 | <P> | |
77 | A similar mechanism is needed by the <tt class="method">__contains__()</tt> and | |
78 | <tt class="method">remove()</tt> methods which need to hash an element to check | |
79 | for membership in a set. Those methods check an element for hashability | |
80 | and, if not, check for a <tt class="method">__as_temporarily_immutable__()</tt> method | |
81 | which returns the element wrapped by a class that provides temporary | |
82 | methods for <tt class="method">__hash__()</tt>, <tt class="method">__eq__()</tt>, and <tt class="method">__ne__()</tt>. | |
83 | ||
84 | <P> | |
85 | The alternate mechanism spares the need to build a separate copy of | |
86 | the original mutable object. | |
87 | ||
88 | <P> | |
89 | <tt class="class">Set</tt> objects implement the <tt class="method">__as_temporarily_immutable__()</tt> | |
90 | method which returns the <tt class="class">Set</tt> object wrapped by a new class | |
91 | <tt class="class">_TemporarilyImmutableSet</tt>. | |
92 | ||
93 | <P> | |
94 | The two mechanisms for adding hashability are normally invisible to the | |
95 | user; however, a conflict can arise in a multi-threaded environment | |
96 | where one thread is updating a set while another has temporarily wrapped it | |
97 | in <tt class="class">_TemporarilyImmutableSet</tt>. In other words, sets of mutable sets | |
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