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55 | <H2><A NAME="SECTION004380000000000000000"></A><A NAME="typesmapping"></A> | |
56 | <BR> | |
57 | 2.3.8 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> | |
61 | A <i class="dfn">mapping</i> object maps immutable values to | |
62 | arbitrary objects. Mappings are mutable objects. There is currently | |
63 | only one standard mapping type, the <i class="dfn">dictionary</i>. A dictionary's keys are | |
64 | almost arbitrary values. Only values containing lists, dictionaries | |
65 | or other mutable types (that are compared by value rather than by | |
66 | object identity) may not be used as keys. | |
67 | Numeric types used for keys obey the normal rules for numeric | |
68 | comparison: 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 | |
70 | dictionary entry. | |
71 | ||
72 | <P> | |
73 | Dictionaries 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> | |
79 | The 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 | |
81 | arbitrary 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> | |
162 | Notes: | |
163 | <DL> | |
164 | <DT><STRONG>(1)</STRONG></DT> | |
165 | <DD>Raises a <tt class="exception">KeyError</tt> exception if <var>k</var> is not | |
166 | in 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 | |
178 | non-random, varies across Python implementations, and depends on the | |
179 | dictionary's history of insertions and deletions. | |
180 | If <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> | |
182 | are called with no intervening modifications to the dictionary, the | |
183 | lists 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 | |
186 | relationship 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>. | |
189 | Another way to create the same list is "<tt class="samp">pairs = [(v, k) for (k, | |
190 | v) 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, | |
196 | instead it returns <var>x</var>. <var>x</var> is optional; when <var>x</var> is not | |
197 | provided 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 | |
203 | that if <var>k</var> is missing, <var>x</var> is both returned and inserted into | |
204 | the 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 | |
210 | over a dictionary, as often used in set algorithms. If the dictionary | |
211 | is 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 | |
217 | new 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 | |
224 | value 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 | |
231 | or an iterable of key/value pairs (as a tuple or other iterable of | |
232 | length two). If keyword arguments are specified, the mapping is | |
233 | then 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: | |
237 | Allowed the argument to be an iterable of key/value | |
238 | pairs and allowed keyword arguments.</span> | |
239 | ||
240 | <P> | |
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