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55<H2><A NAME="SECTION004380000000000000000"></A><A NAME="typesmapping"></A>
56<BR>
572.3.8 Mapping Types -- classdict
58</H2>
59<a id='l2h-227' xml:id='l2h-227'></a><a id='l2h-228' xml:id='l2h-228'></a>
60<P>
61A <i class="dfn">mapping</i> object maps immutable values to
62arbitrary objects. Mappings are mutable objects. There is currently
63only one standard mapping type, the <i class="dfn">dictionary</i>. A dictionary's keys are
64almost arbitrary values. Only values containing lists, dictionaries
65or other mutable types (that are compared by value rather than by
66object identity) may not be used as keys.
67Numeric types used for keys obey the normal rules for numeric
68comparison: if two numbers compare equal (such as <code>1</code> and
69<code>1.0</code>) then they can be used interchangeably to index the same
70dictionary entry.
71
72<P>
73Dictionaries are created by placing a comma-separated list of
74<code><var>key</var>: <var>value</var></code> pairs within braces, for example:
75<code>{'jack': 4098, 'sjoerd': 4127}</code> or
76<code>{4098: 'jack', 4127: 'sjoerd'}</code>.
77
78<P>
79The following operations are defined on mappings (where <var>a</var> and
80<var>b</var> are mappings, <var>k</var> is a key, and <var>v</var> and <var>x</var> are
81arbitrary objects):
82<a id='l2h-229' xml:id='l2h-229'></a><a id='l2h-230' xml:id='l2h-230'></a><a id='l2h-231' xml:id='l2h-231'></a><a id='l2h-232' xml:id='l2h-232'></a><a id='l2h-234' xml:id='l2h-234'></a>
83<P>
84<div class="center"><table class="realtable">
85 <thead>
86 <tr>
87 <th class="center">Operation</th>
88 <th class="left" >Result</th>
89 <th class="center">Notes</th>
90 </tr>
91 </thead>
92 <tbody>
93 <tr><td class="center" valign="baseline"><code>len(<var>a</var>)</code></td>
94 <td class="left" >the number of items in <var>a</var></td>
95 <td class="center"></td></tr>
96 <tr><td class="center" valign="baseline"><code><var>a</var>[<var>k</var>]</code></td>
97 <td class="left" >the item of <var>a</var> with key <var>k</var></td>
98 <td class="center">(1)</td></tr>
99 <tr><td class="center" valign="baseline"><code><var>a</var>[<var>k</var>] = <var>v</var></code></td>
100 <td class="left" >set <code><var>a</var>[<var>k</var>]</code> to <var>v</var></td>
101 <td class="center"></td></tr>
102 <tr><td class="center" valign="baseline"><code>del <var>a</var>[<var>k</var>]</code></td>
103 <td class="left" >remove <code><var>a</var>[<var>k</var>]</code> from <var>a</var></td>
104 <td class="center">(1)</td></tr>
105 <tr><td class="center" valign="baseline"><code><var>a</var>.clear()</code></td>
106 <td class="left" >remove all items from <code>a</code></td>
107 <td class="center"></td></tr>
108 <tr><td class="center" valign="baseline"><code><var>a</var>.copy()</code></td>
109 <td class="left" >a (shallow) copy of <code>a</code></td>
110 <td class="center"></td></tr>
111 <tr><td class="center" valign="baseline"><code><var>a</var>.has_key(<var>k</var>)</code></td>
112 <td class="left" ><code>True</code> if <var>a</var> has a key <var>k</var>, else <code>False</code></td>
113 <td class="center"></td></tr>
114 <tr><td class="center" valign="baseline"><code><var>k</var> <code>in</code> <var>a</var></code></td>
115 <td class="left" >Equivalent to <var>a</var>.has_key(<var>k</var>)</td>
116 <td class="center">(2)</td></tr>
117 <tr><td class="center" valign="baseline"><code><var>k</var> not in <var>a</var></code></td>
118 <td class="left" >Equivalent to <code>not</code> <var>a</var>.has_key(<var>k</var>)</td>
119 <td class="center">(2)</td></tr>
120 <tr><td class="center" valign="baseline"><code><var>a</var>.items()</code></td>
121 <td class="left" >a copy of <var>a</var>'s list of (<var>key</var>, <var>value</var>) pairs</td>
122 <td class="center">(3)</td></tr>
123 <tr><td class="center" valign="baseline"><code><var>a</var>.keys()</code></td>
124 <td class="left" >a copy of <var>a</var>'s list of keys</td>
125 <td class="center">(3)</td></tr>
126 <tr><td class="center" valign="baseline"><code><var>a</var>.update(<big>[</big><var>b</var><big>]</big>)</code></td>
127 <td class="left" >updates (and overwrites) key/value pairs from <var>b</var></td>
128 <td class="center">(9)</td></tr>
129 <tr><td class="center" valign="baseline"><code><var>a</var>.fromkeys(<var>seq</var><big>[</big>, <var>value</var><big>]</big>)</code></td>
130 <td class="left" >Creates a new dictionary with keys from <var>seq</var> and values set to <var>value</var></td>
131 <td class="center">(7)</td></tr>
132 <tr><td class="center" valign="baseline"><code><var>a</var>.values()</code></td>
133 <td class="left" >a copy of <var>a</var>'s list of values</td>
134 <td class="center">(3)</td></tr>
135 <tr><td class="center" valign="baseline"><code><var>a</var>.get(<var>k</var><big>[</big>, <var>x</var><big>]</big>)</code></td>
136 <td class="left" ><code><var>a</var>[<var>k</var>]</code> if <code><var>k</var> in <var>a</var></code>,
137 else <var>x</var></td>
138 <td class="center">(4)</td></tr>
139 <tr><td class="center" valign="baseline"><code><var>a</var>.setdefault(<var>k</var><big>[</big>, <var>x</var><big>]</big>)</code></td>
140 <td class="left" ><code><var>a</var>[<var>k</var>]</code> if <code><var>k</var> in <var>a</var></code>,
141 else <var>x</var> (also setting it)</td>
142 <td class="center">(5)</td></tr>
143 <tr><td class="center" valign="baseline"><code><var>a</var>.pop(<var>k</var><big>[</big>, <var>x</var><big>]</big>)</code></td>
144 <td class="left" ><code><var>a</var>[<var>k</var>]</code> if <code><var>k</var> in <var>a</var></code>,
145 else <var>x</var> (and remove k)</td>
146 <td class="center">(8)</td></tr>
147 <tr><td class="center" valign="baseline"><code><var>a</var>.popitem()</code></td>
148 <td class="left" >remove and return an arbitrary (<var>key</var>, <var>value</var>) pair</td>
149 <td class="center">(6)</td></tr>
150 <tr><td class="center" valign="baseline"><code><var>a</var>.iteritems()</code></td>
151 <td class="left" >return an iterator over (<var>key</var>, <var>value</var>) pairs</td>
152 <td class="center">(2), (3)</td></tr>
153 <tr><td class="center" valign="baseline"><code><var>a</var>.iterkeys()</code></td>
154 <td class="left" >return an iterator over the mapping's keys</td>
155 <td class="center">(2), (3)</td></tr>
156 <tr><td class="center" valign="baseline"><code><var>a</var>.itervalues()</code></td>
157 <td class="left" >return an iterator over the mapping's values</td>
158 <td class="center">(2), (3)</td></tr></tbody>
159</table></div>
160
161<P>
162Notes:
163<DL>
164<DT><STRONG>(1)</STRONG></DT>
165<DD>Raises a <tt class="exception">KeyError</tt> exception if <var>k</var> is not
166in the map.
167
168<P>
169</DD>
170<DT><STRONG>(2)</STRONG></DT>
171<DD>
172<span class="versionnote">New in version 2.2.</span>
173
174<P>
175</DD>
176<DT><STRONG>(3)</STRONG></DT>
177<DD>Keys and values are listed in an arbitrary order which is
178non-random, varies across Python implementations, and depends on the
179dictionary's history of insertions and deletions.
180If <tt class="method">items()</tt>, <tt class="method">keys()</tt>, <tt class="method">values()</tt>,
181<tt class="method">iteritems()</tt>, <tt class="method">iterkeys()</tt>, and <tt class="method">itervalues()</tt>
182are called with no intervening modifications to the dictionary, the
183lists will directly correspond. This allows the creation of
184<code>(<var>value</var>, <var>key</var>)</code> pairs using <tt class="function">zip()</tt>:
185"<tt class="samp">pairs = zip(<var>a</var>.values(), <var>a</var>.keys())</tt>". The same
186relationship holds for the <tt class="method">iterkeys()</tt> and
187<tt class="method">itervalues()</tt> methods: "<tt class="samp">pairs = zip(<var>a</var>.itervalues(),
188<var>a</var>.iterkeys())</tt>" provides the same value for <code>pairs</code>.
189Another way to create the same list is "<tt class="samp">pairs = [(v, k) for (k,
190v) in <var>a</var>.iteritems()]</tt>".
191
192<P>
193</DD>
194<DT><STRONG>(4)</STRONG></DT>
195<DD>Never raises an exception if <var>k</var> is not in the map,
196instead it returns <var>x</var>. <var>x</var> is optional; when <var>x</var> is not
197provided and <var>k</var> is not in the map, <code>None</code> is returned.
198
199<P>
200</DD>
201<DT><STRONG>(5)</STRONG></DT>
202<DD><tt class="function">setdefault()</tt> is like <tt class="function">get()</tt>, except
203that if <var>k</var> is missing, <var>x</var> is both returned and inserted into
204the dictionary as the value of <var>k</var>. <var>x</var> defaults to <var>None</var>.
205
206<P>
207</DD>
208<DT><STRONG>(6)</STRONG></DT>
209<DD><tt class="function">popitem()</tt> is useful to destructively iterate
210over a dictionary, as often used in set algorithms. If the dictionary
211is empty, calling <tt class="function">popitem()</tt> raises a <tt class="exception">KeyError</tt>.
212
213<P>
214</DD>
215<DT><STRONG>(7)</STRONG></DT>
216<DD><tt class="function">fromkeys()</tt> is a class method that returns a
217new dictionary. <var>value</var> defaults to <code>None</code>.
218<span class="versionnote">New in version 2.3.</span>
219
220<P>
221</DD>
222<DT><STRONG>(8)</STRONG></DT>
223<DD><tt class="function">pop()</tt> raises a <tt class="exception">KeyError</tt> when no default
224value is given and the key is not found.
225<span class="versionnote">New in version 2.3.</span>
226
227<P>
228</DD>
229<DT><STRONG>(9)</STRONG></DT>
230<DD><tt class="function">update()</tt> accepts either another mapping object
231or an iterable of key/value pairs (as a tuple or other iterable of
232length two). If keyword arguments are specified, the mapping is
233then is updated with those key/value pairs:
234"<tt class="samp">d.update(red=1, blue=2)</tt>".
235
236<span class="versionnote">Changed in version 2.4:
237Allowed the argument to be an iterable of key/value
238 pairs and allowed keyword arguments.</span>
239
240<P>
241</DD>
242</DL>
243
244<P>
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