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<H2><A NAME="SECTION005350000000000000000"></A><A NAME="sequence-types"></A>
<BR>
3.3.5 Emulating container types
</H2>

<P>
The following methods can be defined to implement container
objects.  Containers usually are sequences (such as lists or tuples)
or mappings (like dictionaries), but can represent other containers as
well.  The first set of methods is used either to emulate a
sequence or to emulate a mapping; the difference is that for a
sequence, the allowable keys should be the integers <var>k</var> for which
<code>0 &lt;= <var>k</var> &lt; <var>N</var></code> where <var>N</var> is the length of the
sequence, or slice objects, which define a range of items. (For backwards
compatibility, the method <tt class="method">__getslice__()</tt> (see below) can also be
defined to handle simple, but not extended slices.) It is also recommended
that mappings provide the methods <tt class="method">keys()</tt>, <tt class="method">values()</tt>,
<tt class="method">items()</tt>, <tt class="method">has_key()</tt>, <tt class="method">get()</tt>, <tt class="method">clear()</tt>,
<tt class="method">setdefault()</tt>, <tt class="method">iterkeys()</tt>, <tt class="method">itervalues()</tt>,
<tt class="method">iteritems()</tt>, <tt class="method">pop()</tt>, <tt class="method">popitem()</tt>,		     
<tt class="method">copy()</tt>, and <tt class="method">update()</tt> behaving similar to those for
Python's standard dictionary objects.  The <tt class="module">UserDict</tt> module
provides a <tt class="class">DictMixin</tt> class to help create those methods
from a base set of <tt class="method">__getitem__()</tt>, <tt class="method">__setitem__()</tt>,
<tt class="method">__delitem__()</tt>, and <tt class="method">keys()</tt>.		     
Mutable sequences should provide
methods <tt class="method">append()</tt>, <tt class="method">count()</tt>, <tt class="method">index()</tt>,
<tt class="method">extend()</tt>,		     
<tt class="method">insert()</tt>, <tt class="method">pop()</tt>, <tt class="method">remove()</tt>, <tt class="method">reverse()</tt>
and <tt class="method">sort()</tt>, like Python standard list objects.  Finally,
sequence types should implement addition (meaning concatenation) and
multiplication (meaning repetition) by defining the methods
<tt class="method">__add__()</tt>, <tt class="method">__radd__()</tt>, <tt class="method">__iadd__()</tt>,
<tt class="method">__mul__()</tt>, <tt class="method">__rmul__()</tt> and <tt class="method">__imul__()</tt> described
below; they should not define <tt class="method">__coerce__()</tt> or other numerical
operators.  It is recommended that both mappings and sequences
implement the <tt class="method">__contains__()</tt> method to allow efficient use of
the <code>in</code> operator; for mappings, <code>in</code> should be equivalent
of <tt class="method">has_key()</tt>; for sequences, it should search through the
values.  It is further recommended that both mappings and sequences
implement the <tt class="method">__iter__()</tt> method to allow efficient iteration
through the container; for mappings, <tt class="method">__iter__()</tt> should be
the same as <tt class="method">iterkeys()</tt>; for sequences, it should iterate
through the values.
<a id='l2h-222' xml:id='l2h-222'></a><a id='l2h-224' xml:id='l2h-224'></a>		     
<a id='l2h-226' xml:id='l2h-226'></a>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-227' xml:id='l2h-227' class="method">__len__</tt></b>(</nobr></td>
  <td><var>self</var>)</td></tr></table></dt>
<dd>
Called to implement the built-in function
<tt class="function">len()</tt><a id='l2h-228' xml:id='l2h-228'></a>.  Should return the length of the
object, an integer <code>&gt;=</code> 0.  Also, an object that doesn't define a
<tt class="method">__nonzero__()</tt> method and whose <tt class="method">__len__()</tt> method
returns zero is considered to be false in a Boolean context.
<a id='l2h-230' xml:id='l2h-230'></a></dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-231' xml:id='l2h-231' class="method">__getitem__</tt></b>(</nobr></td>
  <td><var>self, key</var>)</td></tr></table></dt>
<dd>
Called to implement evaluation of <code><var>self</var>[<var>key</var>]</code>.
For sequence types, the accepted keys should be integers and slice
objects.<a id='l2h-232' xml:id='l2h-232'></a>  Note that
the special interpretation of negative indexes (if the class wishes to
emulate a sequence type) is up to the <tt class="method">__getitem__()</tt> method.
If <var>key</var> is of an inappropriate type, <tt class="exception">TypeError</tt> may be
raised; if of a value outside the set of indexes for the sequence
(after any special interpretation of negative values),
<tt class="exception">IndexError</tt> should be raised.
For mapping types, if <var>key</var> is missing (not in the container),
<tt class="exception">KeyError</tt> should be raised.                     
<span class="note"><b class="label">Note:</b>
<tt class="keyword">for</tt> loops expect that an
<tt class="exception">IndexError</tt> will be raised for illegal indexes to allow
proper detection of the end of the sequence.</span>
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-233' xml:id='l2h-233' class="method">__setitem__</tt></b>(</nobr></td>
  <td><var>self, key, value</var>)</td></tr></table></dt>
<dd>
Called to implement assignment to <code><var>self</var>[<var>key</var>]</code>.  Same
note as for <tt class="method">__getitem__()</tt>.  This should only be implemented
for mappings if the objects support changes to the values for keys, or
if new keys can be added, or for sequences if elements can be
replaced.  The same exceptions should be raised for improper
<var>key</var> values as for the <tt class="method">__getitem__()</tt> method.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-234' xml:id='l2h-234' class="method">__delitem__</tt></b>(</nobr></td>
  <td><var>self, key</var>)</td></tr></table></dt>
<dd>
Called to implement deletion of <code><var>self</var>[<var>key</var>]</code>.  Same
note as for <tt class="method">__getitem__()</tt>.  This should only be implemented
for mappings if the objects support removal of keys, or for sequences
if elements can be removed from the sequence.  The same exceptions
should be raised for improper <var>key</var> values as for the
<tt class="method">__getitem__()</tt> method.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-235' xml:id='l2h-235' class="method">__iter__</tt></b>(</nobr></td>
  <td><var>self</var>)</td></tr></table></dt>
<dd>
This method is called when an iterator is required for a container.
This method should return a new iterator object that can iterate over
all the objects in the container.  For mappings, it should iterate
over the keys of the container, and should also be made available as
the method <tt class="method">iterkeys()</tt>.

<P>
Iterator objects also need to implement this method; they are required
to return themselves.  For more information on iterator objects, see
``<a class="ulink" href="../lib/typeiter.html"
  >Iterator Types</a>'' in the
<em class="citetitle"><a
 href="../lib/lib.html"
 title="Python Library Reference"
 >Python Library Reference</a></em>.
</dl>

<P>
The membership test operators (<tt class="keyword">in</tt> and <tt class="keyword">not in</tt>) are
normally implemented as an iteration through a sequence.  However,
container objects can supply the following special method with a more
efficient implementation, which also does not require the object be a
sequence.

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-236' xml:id='l2h-236' class="method">__contains__</tt></b>(</nobr></td>
  <td><var>self, item</var>)</td></tr></table></dt>
<dd>
Called to implement membership test operators.  Should return true if
<var>item</var> is in <var>self</var>, false otherwise.  For mapping objects,
this should consider the keys of the mapping rather than the values or
the key-item pairs.
</dl>

<P>

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	54
	55	<H2><A NAME="SECTION005350000000000000000"></A><A NAME="sequence-types"></A>
	56	<BR>
	57	3.3.5 Emulating container types
	58	</H2>
	59
	60	<P>
	61	The following methods can be defined to implement container
	62	objects. Containers usually are sequences (such as lists or tuples)
	63	or mappings (like dictionaries), but can represent other containers as
	64	well. The first set of methods is used either to emulate a
65	sequence or to emulate a mapping; the difference is that for a
66	sequence, the allowable keys should be the integers <var>k</var> for which
67	<code>0 <= <var>k</var> < <var>N</var></code> where <var>N</var> is the length of the
68	sequence, or slice objects, which define a range of items. (For backwards
69	compatibility, the method <tt class="method">__getslice__()</tt> (see below) can also be
70	defined to handle simple, but not extended slices.) It is also recommended
71	that mappings provide the methods <tt class="method">keys()</tt>, <tt class="method">values()</tt>,
72	<tt class="method">items()</tt>, <tt class="method">has_key()</tt>, <tt class="method">get()</tt>, <tt class="method">clear()</tt>,
73	<tt class="method">setdefault()</tt>, <tt class="method">iterkeys()</tt>, <tt class="method">itervalues()</tt>,
74	<tt class="method">iteritems()</tt>, <tt class="method">pop()</tt>, <tt class="method">popitem()</tt>,
75	<tt class="method">copy()</tt>, and <tt class="method">update()</tt> behaving similar to those for
76	Python's standard dictionary objects. The <tt class="module">UserDict</tt> module
77	provides a <tt class="class">DictMixin</tt> class to help create those methods
78	from a base set of <tt class="method">__getitem__()</tt>, <tt class="method">__setitem__()</tt>,
79	<tt class="method">__delitem__()</tt>, and <tt class="method">keys()</tt>.
80	Mutable sequences should provide
81	methods <tt class="method">append()</tt>, <tt class="method">count()</tt>, <tt class="method">index()</tt>,
82	<tt class="method">extend()</tt>,
83	<tt class="method">insert()</tt>, <tt class="method">pop()</tt>, <tt class="method">remove()</tt>, <tt class="method">reverse()</tt>
84	and <tt class="method">sort()</tt>, like Python standard list objects. Finally,
85	sequence types should implement addition (meaning concatenation) and
86	multiplication (meaning repetition) by defining the methods
87	<tt class="method">__add__()</tt>, <tt class="method">__radd__()</tt>, <tt class="method">__iadd__()</tt>,
88	<tt class="method">__mul__()</tt>, <tt class="method">__rmul__()</tt> and <tt class="method">__imul__()</tt> described
89	below; they should not define <tt class="method">__coerce__()</tt> or other numerical
90	operators. It is recommended that both mappings and sequences
91	implement the <tt class="method">__contains__()</tt> method to allow efficient use of
92	the <code>in</code> operator; for mappings, <code>in</code> should be equivalent
93	of <tt class="method">has_key()</tt>; for sequences, it should search through the
94	values. It is further recommended that both mappings and sequences
95	implement the <tt class="method">__iter__()</tt> method to allow efficient iteration
96	through the container; for mappings, <tt class="method">__iter__()</tt> should be
97	the same as <tt class="method">iterkeys()</tt>; for sequences, it should iterate
98	through the values.
99	<a id='l2h-222' xml:id='l2h-222'></a><a id='l2h-224' xml:id='l2h-224'></a>
100	<a id='l2h-226' xml:id='l2h-226'></a>
101	<P>
102	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
103	<td><nobr><b><tt id='l2h-227' xml:id='l2h-227' class="method">__len__</tt></b>(</nobr></td>
104	<td><var>self</var>)</td></tr></table></dt>
105	<dd>
106	Called to implement the built-in function
107	<tt class="function">len()</tt><a id='l2h-228' xml:id='l2h-228'></a>. Should return the length of the
108	object, an integer <code>>=</code> 0. Also, an object that doesn't define a
109	<tt class="method">__nonzero__()</tt> method and whose <tt class="method">__len__()</tt> method
110	returns zero is considered to be false in a Boolean context.
111	<a id='l2h-230' xml:id='l2h-230'></a></dl>
112
113	<P>
114	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
115	<td><nobr><b><tt id='l2h-231' xml:id='l2h-231' class="method">__getitem__</tt></b>(</nobr></td>
116	<td><var>self, key</var>)</td></tr></table></dt>
117	<dd>
118	Called to implement evaluation of <code><var>self</var>[<var>key</var>]</code>.
119	For sequence types, the accepted keys should be integers and slice
120	objects.<a id='l2h-232' xml:id='l2h-232'></a> Note that
121	the special interpretation of negative indexes (if the class wishes to
122	emulate a sequence type) is up to the <tt class="method">__getitem__()</tt> method.
123	If <var>key</var> is of an inappropriate type, <tt class="exception">TypeError</tt> may be
124	raised; if of a value outside the set of indexes for the sequence
125	(after any special interpretation of negative values),
126	<tt class="exception">IndexError</tt> should be raised.
127	For mapping types, if <var>key</var> is missing (not in the container),
128	<tt class="exception">KeyError</tt> should be raised.
129	<span class="note"><b class="label">Note:</b>
130	<tt class="keyword">for</tt> loops expect that an
131	<tt class="exception">IndexError</tt> will be raised for illegal indexes to allow
132	proper detection of the end of the sequence.</span>
133	</dl>
134
135	<P>
136	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
137	<td><nobr><b><tt id='l2h-233' xml:id='l2h-233' class="method">__setitem__</tt></b>(</nobr></td>
138	<td><var>self, key, value</var>)</td></tr></table></dt>
139	<dd>
140	Called to implement assignment to <code><var>self</var>[<var>key</var>]</code>. Same
141	note as for <tt class="method">__getitem__()</tt>. This should only be implemented
142	for mappings if the objects support changes to the values for keys, or
143	if new keys can be added, or for sequences if elements can be
144	replaced. The same exceptions should be raised for improper
145	<var>key</var> values as for the <tt class="method">__getitem__()</tt> method.
146	</dl>
147
148	<P>
149	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
150	<td><nobr><b><tt id='l2h-234' xml:id='l2h-234' class="method">__delitem__</tt></b>(</nobr></td>
151	<td><var>self, key</var>)</td></tr></table></dt>
152	<dd>
153	Called to implement deletion of <code><var>self</var>[<var>key</var>]</code>. Same
154	note as for <tt class="method">__getitem__()</tt>. This should only be implemented
155	for mappings if the objects support removal of keys, or for sequences
156	if elements can be removed from the sequence. The same exceptions
157	should be raised for improper <var>key</var> values as for the
158	<tt class="method">__getitem__()</tt> method.
159	</dl>
160
161	<P>
162	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
163	<td><nobr><b><tt id='l2h-235' xml:id='l2h-235' class="method">__iter__</tt></b>(</nobr></td>
164	<td><var>self</var>)</td></tr></table></dt>
165	<dd>
166	This method is called when an iterator is required for a container.
167	This method should return a new iterator object that can iterate over
168	all the objects in the container. For mappings, it should iterate
169	over the keys of the container, and should also be made available as
170	the method <tt class="method">iterkeys()</tt>.
171
172	<P>
173	Iterator objects also need to implement this method; they are required
174	to return themselves. For more information on iterator objects, see
175	``<a class="ulink" href="../lib/typeiter.html"
176	>Iterator Types</a>'' in the
177	<em class="citetitle"><a
178	href="../lib/lib.html"
179	title="Python Library Reference"
180	>Python Library Reference</a></em>.
181	</dl>
182
183	<P>
184	The membership test operators (<tt class="keyword">in</tt> and <tt class="keyword">not in</tt>) are
185	normally implemented as an iteration through a sequence. However,
186	container objects can supply the following special method with a more
187	efficient implementation, which also does not require the object be a
188	sequence.
189
190	<P>
191	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
192	<td><nobr><b><tt id='l2h-236' xml:id='l2h-236' class="method">__contains__</tt></b>(</nobr></td>
193	<td><var>self, item</var>)</td></tr></table></dt>
194	<dd>
195	Called to implement membership test operators. Should return true if
196	<var>item</var> is in <var>self</var>, false otherwise. For mapping objects,
197	this should consider the keys of the mapping rather than the values or
198	the key-item pairs.
199	</dl>
200
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