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| 55 | <H2><A NAME="SECTION0071510000000000000000"></A><A NAME="set-objects"></A> | |
| 56 | <BR> | |
| 57 | 5.15.1 Set Objects | |
| 58 | </H2> | |
| 59 | ||
| 60 | <P> | |
| 61 | Instances of <tt class="class">Set</tt> and <tt class="class">ImmutableSet</tt> both provide | |
| 62 | the following operations: | |
| 63 | ||
| 64 | <P> | |
| 65 | <div class="center"><table class="realtable"> | |
| 66 | <thead> | |
| 67 | <tr> | |
| 68 | <th class="center">Operation</th> | |
| 69 | <th class="center">Equivalent</th> | |
| 70 | <th class="left" >Result</th> | |
| 71 | </tr> | |
| 72 | </thead> | |
| 73 | <tbody> | |
| 74 | <tr><td class="center" valign="baseline"><code>len(<var>s</var>)</code></td> | |
| 75 | <td class="center"></td> | |
| 76 | <td class="left" >cardinality of set <var>s</var></td></tr><P> | |
| 77 | ||
| 78 | <tr><td class="center" valign="baseline"><code><var>x</var> in <var>s</var></code></td> | |
| 79 | <td class="center"></td> | |
| 80 | <td class="left" >test <var>x</var> for membership in <var>s</var></td></tr> | |
| 81 | <tr><td class="center" valign="baseline"><code><var>x</var> not in <var>s</var></code></td> | |
| 82 | <td class="center"></td> | |
| 83 | <td class="left" >test <var>x</var> for non-membership in <var>s</var></td></tr> | |
| 84 | <tr><td class="center" valign="baseline"><code><var>s</var>.issubset(<var>t</var>)</code></td> | |
| 85 | <td class="center"><code><var>s</var> <= <var>t</var></code></td> | |
| 86 | <td class="left" >test whether every element in <var>s</var> is in <var>t</var></td></tr> | |
| 87 | <tr><td class="center" valign="baseline"><code><var>s</var>.issuperset(<var>t</var>)</code></td> | |
| 88 | <td class="center"><code><var>s</var> >= <var>t</var></code></td> | |
| 89 | <td class="left" >test whether every element in <var>t</var> is in <var>s</var></td></tr><P> | |
| 90 | ||
| 91 | <tr><td class="center" valign="baseline"><code><var>s</var>.union(<var>t</var>)</code></td> | |
| 92 | <td class="center"><var>s</var> | <var>t</var></td> | |
| 93 | <td class="left" >new set with elements from both <var>s</var> and <var>t</var></td></tr> | |
| 94 | <tr><td class="center" valign="baseline"><code><var>s</var>.intersection(<var>t</var>)</code></td> | |
| 95 | <td class="center"><var>s</var> & <var>t</var></td> | |
| 96 | <td class="left" >new set with elements common to <var>s</var> and <var>t</var></td></tr> | |
| 97 | <tr><td class="center" valign="baseline"><code><var>s</var>.difference(<var>t</var>)</code></td> | |
| 98 | <td class="center"><var>s</var> - <var>t</var></td> | |
| 99 | <td class="left" >new set with elements in <var>s</var> but not in <var>t</var></td></tr> | |
| 100 | <tr><td class="center" valign="baseline"><code><var>s</var>.symmetric_difference(<var>t</var>)</code></td> | |
| 101 | <td class="center"><var>s</var> ^ <var>t</var></td> | |
| 102 | <td class="left" >new set with elements in either <var>s</var> or <var>t</var> but not both</td></tr> | |
| 103 | <tr><td class="center" valign="baseline"><code><var>s</var>.copy()</code></td> | |
| 104 | <td class="center"></td> | |
| 105 | <td class="left" >new set with a shallow copy of <var>s</var></td></tr></tbody> | |
| 106 | </table></div> | |
| 107 | ||
| 108 | <P> | |
| 109 | Note, the non-operator versions of <tt class="method">union()</tt>, | |
| 110 | <tt class="method">intersection()</tt>, <tt class="method">difference()</tt>, and | |
| 111 | <tt class="method">symmetric_difference()</tt> will accept any iterable as an argument. | |
| 112 | In contrast, their operator based counterparts require their arguments to | |
| 113 | be sets. This precludes error-prone constructions like | |
| 114 | <code>Set('abc') & 'cbs'</code> in favor of the more readable | |
| 115 | <code>Set('abc').intersection('cbs')</code>. | |
| 116 | ||
| 117 | <span class="versionnote">Changed in version 2.3.1: | |
| 118 | Formerly all arguments were required to be sets.</span> | |
| 119 | ||
| 120 | <P> | |
| 121 | In addition, both <tt class="class">Set</tt> and <tt class="class">ImmutableSet</tt> | |
| 122 | support set to set comparisons. Two sets are equal if and only if | |
| 123 | every element of each set is contained in the other (each is a subset | |
| 124 | of the other). | |
| 125 | A set is less than another set if and only if the first set is a proper | |
| 126 | subset of the second set (is a subset, but is not equal). | |
| 127 | A set is greater than another set if and only if the first set is a proper | |
| 128 | superset of the second set (is a superset, but is not equal). | |
| 129 | ||
| 130 | <P> | |
| 131 | The subset and equality comparisons do not generalize to a complete | |
| 132 | ordering function. For example, any two disjoint sets are not equal and | |
| 133 | are not subsets of each other, so <em>all</em> of the following return | |
| 134 | <code>False</code>: <code><var>a</var><<var>b</var></code>, <code><var>a</var>==<var>b</var></code>, or | |
| 135 | <code><var>a</var>><var>b</var></code>. | |
| 136 | Accordingly, sets do not implement the <tt class="method">__cmp__</tt> method. | |
| 137 | ||
| 138 | <P> | |
| 139 | Since sets only define partial ordering (subset relationships), the output | |
| 140 | of the <tt class="method">list.sort()</tt> method is undefined for lists of sets. | |
| 141 | ||
| 142 | <P> | |
| 143 | The following table lists operations available in <tt class="class">ImmutableSet</tt> | |
| 144 | but not found in <tt class="class">Set</tt>: | |
| 145 | ||
| 146 | <P> | |
| 147 | <div class="center"><table class="realtable"> | |
| 148 | <thead> | |
| 149 | <tr> | |
| 150 | <th class="center">Operation</th> | |
| 151 | <th class="left" >Result</th> | |
| 152 | </tr> | |
| 153 | </thead> | |
| 154 | <tbody> | |
| 155 | <tr><td class="center" valign="baseline"><code>hash(<var>s</var>)</code></td> | |
| 156 | <td class="left" >returns a hash value for <var>s</var></td></tr></tbody> | |
| 157 | </table></div> | |
| 158 | ||
| 159 | <P> | |
| 160 | The following table lists operations available in <tt class="class">Set</tt> | |
| 161 | but not found in <tt class="class">ImmutableSet</tt>: | |
| 162 | ||
| 163 | <P> | |
| 164 | <div class="center"><table class="realtable"> | |
| 165 | <thead> | |
| 166 | <tr> | |
| 167 | <th class="center">Operation</th> | |
| 168 | <th class="center">Equivalent</th> | |
| 169 | <th class="left" >Result</th> | |
| 170 | </tr> | |
| 171 | </thead> | |
| 172 | <tbody> | |
| 173 | <tr><td class="center" valign="baseline"><code><var>s</var>.update(<var>t</var>)</code></td> | |
| 174 | <td class="center"><var>s</var> |= <var>t</var></td> | |
| 175 | <td class="left" >return set <var>s</var> with elements added from <var>t</var></td></tr> | |
| 176 | <tr><td class="center" valign="baseline"><code><var>s</var>.intersection_update(<var>t</var>)</code></td> | |
| 177 | <td class="center"><var>s</var> &= <var>t</var></td> | |
| 178 | <td class="left" >return set <var>s</var> keeping only elements also found in <var>t</var></td></tr> | |
| 179 | <tr><td class="center" valign="baseline"><code><var>s</var>.difference_update(<var>t</var>)</code></td> | |
| 180 | <td class="center"><var>s</var> -= <var>t</var></td> | |
| 181 | <td class="left" >return set <var>s</var> after removing elements found in <var>t</var></td></tr> | |
| 182 | <tr><td class="center" valign="baseline"><code><var>s</var>.symmetric_difference_update(<var>t</var>)</code></td> | |
| 183 | <td class="center"><var>s</var> ^= <var>t</var></td> | |
| 184 | <td class="left" >return set <var>s</var> with elements from <var>s</var> or <var>t</var> | |
| 185 | but not both</td></tr><P> | |
| 186 | ||
| 187 | <tr><td class="center" valign="baseline"><code><var>s</var>.add(<var>x</var>)</code></td> | |
| 188 | <td class="center"></td> | |
| 189 | <td class="left" >add element <var>x</var> to set <var>s</var></td></tr> | |
| 190 | <tr><td class="center" valign="baseline"><code><var>s</var>.remove(<var>x</var>)</code></td> | |
| 191 | <td class="center"></td> | |
| 192 | <td class="left" >remove <var>x</var> from set <var>s</var>; raises KeyError if not present</td></tr> | |
| 193 | <tr><td class="center" valign="baseline"><code><var>s</var>.discard(<var>x</var>)</code></td> | |
| 194 | <td class="center"></td> | |
| 195 | <td class="left" >removes <var>x</var> from set <var>s</var> if present</td></tr> | |
| 196 | <tr><td class="center" valign="baseline"><code><var>s</var>.pop()</code></td> | |
| 197 | <td class="center"></td> | |
| 198 | <td class="left" >remove and return an arbitrary element from <var>s</var>; raises | |
| 199 | KeyError if empty</td></tr> | |
| 200 | <tr><td class="center" valign="baseline"><code><var>s</var>.clear()</code></td> | |
| 201 | <td class="center"></td> | |
| 202 | <td class="left" >remove all elements from set <var>s</var></td></tr></tbody> | |
| 203 | </table></div> | |
| 204 | ||
| 205 | <P> | |
| 206 | Note, the non-operator versions of <tt class="method">update()</tt>, | |
| 207 | <tt class="method">intersection_update()</tt>, <tt class="method">difference_update()</tt>, and | |
| 208 | <tt class="method">symmetric_difference_update()</tt> will accept any iterable as | |
| 209 | an argument. | |
| 210 | ||
| 211 | <span class="versionnote">Changed in version 2.3.1: | |
| 212 | Formerly all arguments were required to be sets.</span> | |
| 213 | ||
| 214 | <P> | |
| 215 | Also note, the module also includes a <tt class="method">union_update()</tt> method | |
| 216 | which is an alias for <tt class="method">update()</tt>. The method is included for | |
| 217 | backwards compatibility. Programmers should prefer the | |
| 218 | <tt class="method">update()</tt> method because it the one supported by the builtin | |
| 219 | <tt class="class">set()</tt> and <tt class="class">frozenset()</tt> types. | |
| 220 | ||
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