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| 54 | <H3><A NAME="SECTION004364000000000000000"></A><A NAME="typesseq-mutable"></A> |
| 55 | <BR> |
| 56 | 2.3.6.4 Mutable Sequence Types |
| 57 | </H3> |
| 58 | |
| 59 | <P> |
| 60 | List objects support additional operations that allow in-place |
| 61 | modification of the object. |
| 62 | Other mutable sequence types (when added to the language) should |
| 63 | also support these operations. |
| 64 | Strings and tuples are immutable sequence types: such objects cannot |
| 65 | be modified once created. |
| 66 | The following operations are defined on mutable sequence types (where |
| 67 | <var>x</var> is an arbitrary object): |
| 68 | <a id='l2h-215' xml:id='l2h-215'></a><a id='l2h-216' xml:id='l2h-216'></a> |
| 69 | <P> |
| 70 | <div class="center"><table class="realtable"> |
| 71 | <thead> |
| 72 | <tr> |
| 73 | <th class="center">Operation</th> |
| 74 | <th class="left" >Result</th> |
| 75 | <th class="center">Notes</th> |
| 76 | </tr> |
| 77 | </thead> |
| 78 | <tbody> |
| 79 | <tr><td class="center" valign="baseline"><code><var>s</var>[<var>i</var>] = <var>x</var></code></td> |
| 80 | <td class="left" >item <var>i</var> of <var>s</var> is replaced by <var>x</var></td> |
| 81 | <td class="center"></td></tr> |
| 82 | <tr><td class="center" valign="baseline"><code><var>s</var>[<var>i</var>:<var>j</var>] = <var>t</var></code></td> |
| 83 | <td class="left" >slice of <var>s</var> from <var>i</var> to <var>j</var> is replaced by <var>t</var></td> |
| 84 | <td class="center"></td></tr> |
| 85 | <tr><td class="center" valign="baseline"><code>del <var>s</var>[<var>i</var>:<var>j</var>]</code></td> |
| 86 | <td class="left" >same as <code><var>s</var>[<var>i</var>:<var>j</var>] = []</code></td> |
| 87 | <td class="center"></td></tr> |
| 88 | <tr><td class="center" valign="baseline"><code><var>s</var>[<var>i</var>:<var>j</var>:<var>k</var>] = <var>t</var></code></td> |
| 89 | <td class="left" >the elements of <code><var>s</var>[<var>i</var>:<var>j</var>:<var>k</var>]</code> are replaced by those of <var>t</var></td> |
| 90 | <td class="center">(1)</td></tr> |
| 91 | <tr><td class="center" valign="baseline"><code>del <var>s</var>[<var>i</var>:<var>j</var>:<var>k</var>]</code></td> |
| 92 | <td class="left" >removes the elements of <code><var>s</var>[<var>i</var>:<var>j</var>:<var>k</var>]</code> from the list</td> |
| 93 | <td class="center"></td></tr> |
| 94 | <tr><td class="center" valign="baseline"><code><var>s</var>.append(<var>x</var>)</code></td> |
| 95 | <td class="left" >same as <code><var>s</var>[len(<var>s</var>):len(<var>s</var>)] = [<var>x</var>]</code></td> |
| 96 | <td class="center">(2)</td></tr> |
| 97 | <tr><td class="center" valign="baseline"><code><var>s</var>.extend(<var>x</var>)</code></td> |
| 98 | <td class="left" >same as <code><var>s</var>[len(<var>s</var>):len(<var>s</var>)] = <var>x</var></code></td> |
| 99 | <td class="center">(3)</td></tr> |
| 100 | <tr><td class="center" valign="baseline"><code><var>s</var>.count(<var>x</var>)</code></td> |
| 101 | <td class="left" >return number of <var>i</var>'s for which <code><var>s</var>[<var>i</var>] == <var>x</var></code></td> |
| 102 | <td class="center"></td></tr> |
| 103 | <tr><td class="center" valign="baseline"><code><var>s</var>.index(<var>x</var><big>[</big>, <var>i</var><big>[</big>, <var>j</var><big>]</big><big>]</big>)</code></td> |
| 104 | <td class="left" >return smallest <var>k</var> such that <code><var>s</var>[<var>k</var>] == <var>x</var></code> and |
| 105 | <code><var>i</var> <= <var>k</var> < <var>j</var></code></td> |
| 106 | <td class="center">(4)</td></tr> |
| 107 | <tr><td class="center" valign="baseline"><code><var>s</var>.insert(<var>i</var>, <var>x</var>)</code></td> |
| 108 | <td class="left" >same as <code><var>s</var>[<var>i</var>:<var>i</var>] = [<var>x</var>]</code></td> |
| 109 | <td class="center">(5)</td></tr> |
| 110 | <tr><td class="center" valign="baseline"><code><var>s</var>.pop(<big>[</big><var>i</var><big>]</big>)</code></td> |
| 111 | <td class="left" >same as <code><var>x</var> = <var>s</var>[<var>i</var>]; del <var>s</var>[<var>i</var>]; return <var>x</var></code></td> |
| 112 | <td class="center">(6)</td></tr> |
| 113 | <tr><td class="center" valign="baseline"><code><var>s</var>.remove(<var>x</var>)</code></td> |
| 114 | <td class="left" >same as <code>del <var>s</var>[<var>s</var>.index(<var>x</var>)]</code></td> |
| 115 | <td class="center">(4)</td></tr> |
| 116 | <tr><td class="center" valign="baseline"><code><var>s</var>.reverse()</code></td> |
| 117 | <td class="left" >reverses the items of <var>s</var> in place</td> |
| 118 | <td class="center">(7)</td></tr> |
| 119 | <tr><td class="center" valign="baseline"><code><var>s</var>.sort(<big>[</big><var>cmp</var><big>[</big>, |
| 120 | <var>key</var><big>[</big>, <var>reverse</var><big>]</big><big>]</big><big>]</big>)</code></td> |
| 121 | <td class="left" >sort the items of <var>s</var> in place</td> |
| 122 | <td class="center">(7), (8), (9), (10)</td></tr></tbody> |
| 123 | </table></div> |
| 124 | <a id='l2h-217' xml:id='l2h-217'></a><a id='l2h-218' xml:id='l2h-218'></a><a id='l2h-219' xml:id='l2h-219'></a><a id='l2h-220' xml:id='l2h-220'></a><a id='l2h-221' xml:id='l2h-221'></a><a id='l2h-222' xml:id='l2h-222'></a><a id='l2h-223' xml:id='l2h-223'></a><a id='l2h-225' xml:id='l2h-225'></a> |
| 125 | Notes: |
| 126 | <DL> |
| 127 | <DT><STRONG>(1)</STRONG></DT> |
| 128 | <DD><var>t</var> must have the same length as the slice it is |
| 129 | replacing. |
| 130 | |
| 131 | <P> |
| 132 | </DD> |
| 133 | <DT><STRONG>(2)</STRONG></DT> |
| 134 | <DD>The C implementation of Python has historically accepted |
| 135 | multiple parameters and implicitly joined them into a tuple; this |
| 136 | no longer works in Python 2.0. Use of this misfeature has been |
| 137 | deprecated since Python 1.4. |
| 138 | |
| 139 | <P> |
| 140 | </DD> |
| 141 | <DT><STRONG>(3)</STRONG></DT> |
| 142 | <DD><var>x</var> can be any iterable object. |
| 143 | |
| 144 | <P> |
| 145 | </DD> |
| 146 | <DT><STRONG>(4)</STRONG></DT> |
| 147 | <DD>Raises <tt class="exception">ValueError</tt> when <var>x</var> is not found in |
| 148 | <var>s</var>. When a negative index is passed as the second or third parameter |
| 149 | to the <tt class="method">index()</tt> method, the list length is added, as for slice |
| 150 | indices. If it is still negative, it is truncated to zero, as for |
| 151 | slice indices. |
| 152 | <span class="versionnote">Changed in version 2.3: |
| 153 | Previously, <tt class="method">index()</tt> didn't |
| 154 | have arguments for specifying start and stop positions.</span> |
| 155 | |
| 156 | <P> |
| 157 | </DD> |
| 158 | <DT><STRONG>(5)</STRONG></DT> |
| 159 | <DD>When a negative index is passed as the first parameter to |
| 160 | the <tt class="method">insert()</tt> method, the list length is added, as for slice |
| 161 | indices. If it is still negative, it is truncated to zero, as for |
| 162 | slice indices. |
| 163 | <span class="versionnote">Changed in version 2.3: |
| 164 | Previously, all negative indices |
| 165 | were truncated to zero.</span> |
| 166 | |
| 167 | <P> |
| 168 | </DD> |
| 169 | <DT><STRONG>(6)</STRONG></DT> |
| 170 | <DD>The <tt class="method">pop()</tt> method is only supported by the list and |
| 171 | array types. The optional argument <var>i</var> defaults to <code>-1</code>, |
| 172 | so that by default the last item is removed and returned. |
| 173 | |
| 174 | <P> |
| 175 | </DD> |
| 176 | <DT><STRONG>(7)</STRONG></DT> |
| 177 | <DD>The <tt class="method">sort()</tt> and <tt class="method">reverse()</tt> methods modify the |
| 178 | list in place for economy of space when sorting or reversing a large |
| 179 | list. To remind you that they operate by side effect, they don't return |
| 180 | the sorted or reversed list. |
| 181 | |
| 182 | <P> |
| 183 | </DD> |
| 184 | <DT><STRONG>(8)</STRONG></DT> |
| 185 | <DD>The <tt class="method">sort()</tt> method takes optional arguments for |
| 186 | controlling the comparisons. |
| 187 | |
| 188 | <P> |
| 189 | <var>cmp</var> specifies a custom comparison function of two arguments |
| 190 | (list items) which should return a negative, zero or positive number |
| 191 | depending on whether the first argument is considered smaller than, |
| 192 | equal to, or larger than the second argument: |
| 193 | "<tt class="samp"><var>cmp</var>=<tt class="keyword">lambda</tt> <var>x</var>,<var>y</var>: |
| 194 | <tt class="function">cmp</tt>(x.lower(), y.lower())</tt>" |
| 195 | <P> |
| 196 | <var>key</var> specifies a function of one argument that is used to |
| 197 | extract a comparison key from each list element: |
| 198 | "<tt class="samp"><var>key</var>=<tt class="function">str.lower</tt></tt>" |
| 199 | <P> |
| 200 | <var>reverse</var> is a boolean value. If set to <code>True</code>, then the |
| 201 | list elements are sorted as if each comparison were reversed. |
| 202 | |
| 203 | <P> |
| 204 | In general, the <var>key</var> and <var>reverse</var> conversion processes are |
| 205 | much faster than specifying an equivalent <var>cmp</var> function. This is |
| 206 | because <var>cmp</var> is called multiple times for each list element while |
| 207 | <var>key</var> and <var>reverse</var> touch each element only once. |
| 208 | |
| 209 | <P> |
| 210 | |
| 211 | <span class="versionnote">Changed in version 2.3: |
| 212 | Support for <code>None</code> as an equivalent to omitting |
| 213 | <var>cmp</var> was added.</span> |
| 214 | |
| 215 | <P> |
| 216 | |
| 217 | <span class="versionnote">Changed in version 2.4: |
| 218 | Support for <var>key</var> and <var>reverse</var> was added.</span> |
| 219 | |
| 220 | <P> |
| 221 | </DD> |
| 222 | <DT><STRONG>(9)</STRONG></DT> |
| 223 | <DD>Starting with Python 2.3, the <tt class="method">sort()</tt> method is |
| 224 | guaranteed to be stable. A sort is stable if it guarantees not to |
| 225 | change the relative order of elements that compare equal -- this is |
| 226 | helpful for sorting in multiple passes (for example, sort by |
| 227 | department, then by salary grade). |
| 228 | |
| 229 | <P> |
| 230 | </DD> |
| 231 | <DT><STRONG>(10)</STRONG></DT> |
| 232 | <DD>While a list is being sorted, the effect of attempting to |
| 233 | mutate, or even inspect, the list is undefined. The C |
| 234 | implementation of Python 2.3 and newer makes the list appear empty |
| 235 | for the duration, and raises <tt class="exception">ValueError</tt> if it can detect |
| 236 | that the list has been mutated during a sort. |
| 237 | </DD> |
| 238 | </DL> |
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