[OpenSPARC-T2-SAM] / sam-t2 / devtools / v9 / html / python / lib / immutable-transforms.html

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<H2><A NAME="SECTION0071530000000000000000"></A><A NAME="immutable-transforms"></A>
<BR>
5.15.3 Protocol for automatic conversion to immutable
            
</H2>

<P>
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.

<P>
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.

<P>
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
construct sets of sets.

<P>
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>.

<P>
The alternate mechanism spares the need to build a separate copy of
the original mutable object.

<P>
<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>.

<P>
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
are not thread-safe.

<P>

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	54
	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
98	are not thread-safe.
99
100	<P>
101
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