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| 17 | <title>5.15 sets -- Unordered collections of unique elements</title> |
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| 55 | <H1><A NAME="SECTION0071500000000000000000"> |
| 56 | 5.15 <tt class="module">sets</tt> -- |
| 57 | Unordered collections of unique elements</A> |
| 58 | </H1> |
| 59 | |
| 60 | <P> |
| 61 | <A NAME="module-sets"></A> |
| 62 | |
| 63 | <P> |
| 64 | |
| 65 | <span class="versionnote">New in version 2.3.</span> |
| 66 | |
| 67 | <P> |
| 68 | The <tt class="module">sets</tt> module provides classes for constructing and manipulating |
| 69 | unordered collections of unique elements. Common uses include membership |
| 70 | testing, removing duplicates from a sequence, and computing standard math |
| 71 | operations on sets such as intersection, union, difference, and symmetric |
| 72 | difference. |
| 73 | |
| 74 | <P> |
| 75 | Like other collections, sets support <code><var>x</var> in <var>set</var></code>, |
| 76 | <code>len(<var>set</var>)</code>, and <code>for <var>x</var> in <var>set</var></code>. Being an |
| 77 | unordered collection, sets do not record element position or order of |
| 78 | insertion. Accordingly, sets do not support indexing, slicing, or |
| 79 | other sequence-like behavior. |
| 80 | |
| 81 | <P> |
| 82 | Most set applications use the <tt class="class">Set</tt> class which provides every set |
| 83 | method except for <tt class="method">__hash__()</tt>. For advanced applications requiring |
| 84 | a hash method, the <tt class="class">ImmutableSet</tt> class adds a <tt class="method">__hash__()</tt> |
| 85 | method but omits methods which alter the contents of the set. Both |
| 86 | <tt class="class">Set</tt> and <tt class="class">ImmutableSet</tt> derive from <tt class="class">BaseSet</tt>, an |
| 87 | abstract class useful for determining whether something is a set: |
| 88 | <code>isinstance(<var>obj</var>, BaseSet)</code>. |
| 89 | |
| 90 | <P> |
| 91 | The set classes are implemented using dictionaries. Accordingly, the |
| 92 | requirements for set elements are the same as those for dictionary keys; |
| 93 | namely, that the element defines both <tt class="method">__eq__</tt> and <tt class="method">__hash__</tt>. |
| 94 | As a result, sets |
| 95 | cannot contain mutable elements such as lists or dictionaries. |
| 96 | However, they can contain immutable collections such as tuples or |
| 97 | instances of <tt class="class">ImmutableSet</tt>. For convenience in implementing |
| 98 | sets of sets, inner sets are automatically converted to immutable |
| 99 | form, for example, <code>Set([Set(['dog'])])</code> is transformed to |
| 100 | <code>Set([ImmutableSet(['dog'])])</code>. |
| 101 | |
| 102 | <P> |
| 103 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 104 | <td><nobr><b><span class="typelabel">class</span> <tt id='l2h-1380' xml:id='l2h-1380' class="class">Set</tt></b>(</nobr></td> |
| 105 | <td><var></var><big>[</big><var>iterable</var><big>]</big><var></var>)</td></tr></table></dt> |
| 106 | <dd> |
| 107 | Constructs a new empty <tt class="class">Set</tt> object. If the optional <var>iterable</var> |
| 108 | parameter is supplied, updates the set with elements obtained from iteration. |
| 109 | All of the elements in <var>iterable</var> should be immutable or be transformable |
| 110 | to an immutable using the protocol described in |
| 111 | section <A href="immutable-transforms.html#immutable-transforms">5.15.3</A>. |
| 112 | </dl> |
| 113 | |
| 114 | <P> |
| 115 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 116 | <td><nobr><b><span class="typelabel">class</span> <tt id='l2h-1381' xml:id='l2h-1381' class="class">ImmutableSet</tt></b>(</nobr></td> |
| 117 | <td><var></var><big>[</big><var>iterable</var><big>]</big><var></var>)</td></tr></table></dt> |
| 118 | <dd> |
| 119 | Constructs a new empty <tt class="class">ImmutableSet</tt> object. If the optional |
| 120 | <var>iterable</var> parameter is supplied, updates the set with elements obtained |
| 121 | from iteration. All of the elements in <var>iterable</var> should be immutable or |
| 122 | be transformable to an immutable using the protocol described in |
| 123 | section <A href="immutable-transforms.html#immutable-transforms">5.15.3</A>. |
| 124 | |
| 125 | <P> |
| 126 | Because <tt class="class">ImmutableSet</tt> objects provide a <tt class="method">__hash__()</tt> method, |
| 127 | they can be used as set elements or as dictionary keys. <tt class="class">ImmutableSet</tt> |
| 128 | objects do not have methods for adding or removing elements, so all of the |
| 129 | elements must be known when the constructor is called. |
| 130 | </dl> |
| 131 | |
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| 136 | <!--Table of Child-Links--> |
| 137 | <A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a> |
| 138 | |
| 139 | <UL CLASS="ChildLinks"> |
| 140 | <LI><A href="set-objects.html">5.15.1 Set Objects</a> |
| 141 | <LI><A href="set-example.html">5.15.2 Example</a> |
| 142 | <LI><A href="immutable-transforms.html">5.15.3 Protocol for automatic conversion to immutable</a> |
| 143 | <LI><A href="comparison-to-builtin-set.html">5.15.4 Comparison to the built-in <tt class="class">set</tt> types</a> |
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| 182 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> |
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