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| 55 | <H2><A NAME="SECTION005360000000000000000"></A><A NAME="sequence-methods"></A> |
| 56 | <BR> |
| 57 | 3.3.6 Additional methods for emulation of sequence types |
| 58 | |
| 59 | </H2> |
| 60 | |
| 61 | <P> |
| 62 | The following optional methods can be defined to further emulate sequence |
| 63 | objects. Immutable sequences methods should at most only define |
| 64 | <tt class="method">__getslice__()</tt>; mutable sequences might define all three |
| 65 | methods. |
| 66 | |
| 67 | <P> |
| 68 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 69 | <td><nobr><b><tt id='l2h-237' xml:id='l2h-237' class="method">__getslice__</tt></b>(</nobr></td> |
| 70 | <td><var>self, i, j</var>)</td></tr></table></dt> |
| 71 | <dd> |
| 72 | <div class="versionnote"><b>Deprecated since release 2.0.</b> |
| 73 | Support slice objects as parameters to the |
| 74 | <tt class="method">__getitem__()</tt> method.</div><p></p> |
| 75 | Called to implement evaluation of <code><var>self</var>[<var>i</var>:<var>j</var>]</code>. |
| 76 | The returned object should be of the same type as <var>self</var>. Note |
| 77 | that missing <var>i</var> or <var>j</var> in the slice expression are replaced |
| 78 | by zero or <code>sys.maxint</code>, respectively. If negative indexes are |
| 79 | used in the slice, the length of the sequence is added to that index. |
| 80 | If the instance does not implement the <tt class="method">__len__()</tt> method, an |
| 81 | <tt class="exception">AttributeError</tt> is raised. |
| 82 | No guarantee is made that indexes adjusted this way are not still |
| 83 | negative. Indexes which are greater than the length of the sequence |
| 84 | are not modified. |
| 85 | If no <tt class="method">__getslice__()</tt> is found, a slice |
| 86 | object is created instead, and passed to <tt class="method">__getitem__()</tt> instead. |
| 87 | </dl> |
| 88 | |
| 89 | <P> |
| 90 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 91 | <td><nobr><b><tt id='l2h-238' xml:id='l2h-238' class="method">__setslice__</tt></b>(</nobr></td> |
| 92 | <td><var>self, i, j, sequence</var>)</td></tr></table></dt> |
| 93 | <dd> |
| 94 | Called to implement assignment to <code><var>self</var>[<var>i</var>:<var>j</var>]</code>. |
| 95 | Same notes for <var>i</var> and <var>j</var> as for <tt class="method">__getslice__()</tt>. |
| 96 | |
| 97 | <P> |
| 98 | This method is deprecated. If no <tt class="method">__setslice__()</tt> is found, |
| 99 | or for extended slicing of the form |
| 100 | <code><var>self</var>[<var>i</var>:<var>j</var>:<var>k</var>]</code>, a |
| 101 | slice object is created, and passed to <tt class="method">__setitem__()</tt>, |
| 102 | instead of <tt class="method">__setslice__()</tt> being called. |
| 103 | </dl> |
| 104 | |
| 105 | <P> |
| 106 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 107 | <td><nobr><b><tt id='l2h-239' xml:id='l2h-239' class="method">__delslice__</tt></b>(</nobr></td> |
| 108 | <td><var>self, i, j</var>)</td></tr></table></dt> |
| 109 | <dd> |
| 110 | Called to implement deletion of <code><var>self</var>[<var>i</var>:<var>j</var>]</code>. |
| 111 | Same notes for <var>i</var> and <var>j</var> as for <tt class="method">__getslice__()</tt>. |
| 112 | This method is deprecated. If no <tt class="method">__delslice__()</tt> is found, |
| 113 | or for extended slicing of the form |
| 114 | <code><var>self</var>[<var>i</var>:<var>j</var>:<var>k</var>]</code>, a |
| 115 | slice object is created, and passed to <tt class="method">__delitem__()</tt>, |
| 116 | instead of <tt class="method">__delslice__()</tt> being called. |
| 117 | </dl> |
| 118 | |
| 119 | <P> |
| 120 | Notice that these methods are only invoked when a single slice with a |
| 121 | single colon is used, and the slice method is available. For slice |
| 122 | operations involving extended slice notation, or in absence of the |
| 123 | slice methods, <tt class="method">__getitem__()</tt>, <tt class="method">__setitem__()</tt> or |
| 124 | <tt class="method">__delitem__()</tt> is called with a slice object as argument. |
| 125 | |
| 126 | <P> |
| 127 | The following example demonstrate how to make your program or module |
| 128 | compatible with earlier versions of Python (assuming that methods |
| 129 | <tt class="method">__getitem__()</tt>, <tt class="method">__setitem__()</tt> and <tt class="method">__delitem__()</tt> |
| 130 | support slice objects as arguments): |
| 131 | |
| 132 | <P> |
| 133 | <div class="verbatim"><pre> |
| 134 | class MyClass: |
| 135 | ... |
| 136 | def __getitem__(self, index): |
| 137 | ... |
| 138 | def __setitem__(self, index, value): |
| 139 | ... |
| 140 | def __delitem__(self, index): |
| 141 | ... |
| 142 | |
| 143 | if sys.version_info < (2, 0): |
| 144 | # They won't be defined if version is at least 2.0 final |
| 145 | |
| 146 | def __getslice__(self, i, j): |
| 147 | return self[max(0, i):max(0, j):] |
| 148 | def __setslice__(self, i, j, seq): |
| 149 | self[max(0, i):max(0, j):] = seq |
| 150 | def __delslice__(self, i, j): |
| 151 | del self[max(0, i):max(0, j):] |
| 152 | ... |
| 153 | </pre></div> |
| 154 | |
| 155 | <P> |
| 156 | Note the calls to <tt class="function">max()</tt>; these are necessary because of |
| 157 | the handling of negative indices before the |
| 158 | <tt class="method">__*slice__()</tt> methods are called. When negative indexes are |
| 159 | used, the <tt class="method">__*item__()</tt> methods receive them as provided, but |
| 160 | the <tt class="method">__*slice__()</tt> methods get a ``cooked'' form of the index |
| 161 | values. For each negative index value, the length of the sequence is |
| 162 | added to the index before calling the method (which may still result |
| 163 | in a negative index); this is the customary handling of negative |
| 164 | indexes by the built-in sequence types, and the <tt class="method">__*item__()</tt> |
| 165 | methods are expected to do this as well. However, since they should |
| 166 | already be doing that, negative indexes cannot be passed in; they must |
| 167 | be constrained to the bounds of the sequence before being passed to |
| 168 | the <tt class="method">__*item__()</tt> methods. |
| 169 | Calling <code>max(0, i)</code> conveniently returns the proper value. |
| 170 | |
| 171 | <P> |
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