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<H3><A NAME="SECTION004364000000000000000"></A><A NAME="typesseq-mutable"></A>
<BR>
2.3.6.4 Mutable Sequence Types 
</H3>

<P>
List objects support additional operations that allow in-place
modification of the object.
Other mutable sequence types (when added to the language) should
also support these operations.
Strings and tuples are immutable sequence types: such objects cannot
be modified once created.
The following operations are defined on mutable sequence types (where
<var>x</var> is an arbitrary object):
<a id='l2h-215' xml:id='l2h-215'></a><a id='l2h-216' xml:id='l2h-216'></a>
<P>
<div class="center"><table class="realtable">
  <thead>
    <tr>
      <th class="center">Operation</th>
      <th class="left"  >Result</th>
      <th class="center">Notes</th>
      </tr>
    </thead>
  <tbody>
    <tr><td class="center" valign="baseline"><code><var>s</var>[<var>i</var>] = <var>x</var></code></td>
        <td class="left"  >item <var>i</var> of <var>s</var> is replaced by <var>x</var></td>
        <td class="center"></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>[<var>i</var>:<var>j</var>] = <var>t</var></code></td>
        <td class="left"  >slice of <var>s</var> from <var>i</var> to <var>j</var> is replaced by <var>t</var></td>
        <td class="center"></td></tr>
    <tr><td class="center" valign="baseline"><code>del <var>s</var>[<var>i</var>:<var>j</var>]</code></td>
        <td class="left"  >same as <code><var>s</var>[<var>i</var>:<var>j</var>] = []</code></td>
        <td class="center"></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>[<var>i</var>:<var>j</var>:<var>k</var>] = <var>t</var></code></td>
        <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>
        <td class="center">(1)</td></tr>
    <tr><td class="center" valign="baseline"><code>del <var>s</var>[<var>i</var>:<var>j</var>:<var>k</var>]</code></td>
        <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>
        <td class="center"></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.append(<var>x</var>)</code></td>
        <td class="left"  >same as <code><var>s</var>[len(<var>s</var>):len(<var>s</var>)] = [<var>x</var>]</code></td>
        <td class="center">(2)</td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.extend(<var>x</var>)</code></td>
        <td class="left"  >same as <code><var>s</var>[len(<var>s</var>):len(<var>s</var>)] = <var>x</var></code></td>
        <td class="center">(3)</td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.count(<var>x</var>)</code></td>
        <td class="left"  >return number of <var>i</var>'s for which <code><var>s</var>[<var>i</var>] == <var>x</var></code></td>
        <td class="center"></td></tr>
    <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>
        <td class="left"  >return smallest <var>k</var> such that <code><var>s</var>[<var>k</var>] == <var>x</var></code> and
    <code><var>i</var> &lt;= <var>k</var> &lt; <var>j</var></code></td>
        <td class="center">(4)</td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.insert(<var>i</var>, <var>x</var>)</code></td>
        <td class="left"  >same as <code><var>s</var>[<var>i</var>:<var>i</var>] = [<var>x</var>]</code></td>
        <td class="center">(5)</td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.pop(<big>[</big><var>i</var><big>]</big>)</code></td>
        <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>
        <td class="center">(6)</td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.remove(<var>x</var>)</code></td>
        <td class="left"  >same as <code>del <var>s</var>[<var>s</var>.index(<var>x</var>)]</code></td>
        <td class="center">(4)</td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.reverse()</code></td>
        <td class="left"  >reverses the items of <var>s</var> in place</td>
        <td class="center">(7)</td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.sort(<big>[</big><var>cmp</var><big>[</big>,
                        <var>key</var><big>[</big>, <var>reverse</var><big>]</big><big>]</big><big>]</big>)</code></td>
        <td class="left"  >sort the items of <var>s</var> in place</td>
        <td class="center">(7), (8), (9), (10)</td></tr></tbody>
</table></div>
<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>
Notes:
<DL>
<DT><STRONG>(1)</STRONG></DT>
<DD><var>t</var> must have the same length as the slice it is 
  replacing.

<P>
</DD>
<DT><STRONG>(2)</STRONG></DT>
<DD>The C implementation of Python has historically accepted
  multiple parameters and implicitly joined them into a tuple; this
  no longer works in Python 2.0.  Use of this misfeature has been
  deprecated since Python 1.4.

<P>
</DD>
<DT><STRONG>(3)</STRONG></DT>
<DD><var>x</var> can be any iterable object.

<P>
</DD>
<DT><STRONG>(4)</STRONG></DT>
<DD>Raises <tt class="exception">ValueError</tt> when <var>x</var> is not found in
  <var>s</var>. When a negative index is passed as the second or third parameter
  to the <tt class="method">index()</tt> method, the list length is added, as for slice
  indices.  If it is still negative, it is truncated to zero, as for
  slice indices.  
<span class="versionnote">Changed in version 2.3:
Previously, <tt class="method">index()</tt> didn't
  have arguments for specifying start and stop positions.</span>

<P>
</DD>
<DT><STRONG>(5)</STRONG></DT>
<DD>When a negative index is passed as the first parameter to
  the <tt class="method">insert()</tt> method, the list length is added, as for slice
  indices.  If it is still negative, it is truncated to zero, as for
  slice indices.  
<span class="versionnote">Changed in version 2.3:
Previously, all negative indices
  were truncated to zero.</span>

<P>
</DD>
<DT><STRONG>(6)</STRONG></DT>
<DD>The <tt class="method">pop()</tt> method is only supported by the list and
  array types.  The optional argument <var>i</var> defaults to <code>-1</code>,
  so that by default the last item is removed and returned.

<P>
</DD>
<DT><STRONG>(7)</STRONG></DT>
<DD>The <tt class="method">sort()</tt> and <tt class="method">reverse()</tt> methods modify the
  list in place for economy of space when sorting or reversing a large
  list.  To remind you that they operate by side effect, they don't return
  the sorted or reversed list.

<P>
</DD>
<DT><STRONG>(8)</STRONG></DT>
<DD>The <tt class="method">sort()</tt> method takes optional arguments for
  controlling the comparisons.

<P>
<var>cmp</var> specifies a custom comparison function of two arguments
     (list items) which should return a negative, zero or positive number
     depending on whether the first argument is considered smaller than,
     equal to, or larger than the second argument:
     "<tt class="samp"><var>cmp</var>=<tt class="keyword">lambda</tt> <var>x</var>,<var>y</var>:
     <tt class="function">cmp</tt>(x.lower(), y.lower())</tt>"
<P>
<var>key</var> specifies a function of one argument that is used to
     extract a comparison key from each list element:
     "<tt class="samp"><var>key</var>=<tt class="function">str.lower</tt></tt>"
<P>
<var>reverse</var> is a boolean value.  If set to <code>True</code>, then the
     list elements are sorted as if each comparison were reversed.

<P>
In general, the <var>key</var> and <var>reverse</var> conversion processes are
  much faster than specifying an equivalent <var>cmp</var> function.  This is
  because <var>cmp</var> is called multiple times for each list element while
  <var>key</var> and <var>reverse</var> touch each element only once.

<P>

<span class="versionnote">Changed in version 2.3:
Support for <code>None</code> as an equivalent to omitting
  <var>cmp</var> was added.</span>

<P>

<span class="versionnote">Changed in version 2.4:
Support for <var>key</var> and <var>reverse</var> was added.</span>

<P>
</DD>
<DT><STRONG>(9)</STRONG></DT>
<DD>Starting with Python 2.3, the <tt class="method">sort()</tt> method is
  guaranteed to be stable.  A sort is stable if it guarantees not to
  change the relative order of elements that compare equal -- this is
  helpful for sorting in multiple passes (for example, sort by
  department, then by salary grade).

<P>
</DD>
<DT><STRONG>(10)</STRONG></DT>
<DD>While a list is being sorted, the effect of attempting to
  mutate, or even inspect, the list is undefined.  The C
  implementation of Python 2.3 and newer makes the list appear empty
  for the duration, and raises <tt class="exception">ValueError</tt> if it can detect
  that the list has been mutated during a sort.
</DD>
</DL>

<P>

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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>
239
240	<P>
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