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| 54 | ||
| 55 | <H1><A NAME="SECTION005700000000000000000"> | |
| 56 | 3.7 <tt class="module">UserDict</tt> -- | |
| 57 | Class wrapper for dictionary objects</A> | |
| 58 | </H1> | |
| 59 | ||
| 60 | <P> | |
| 61 | <A NAME="module-UserDict"></A> | |
| 62 | ||
| 63 | <P> | |
| 64 | The module defines a mixin, <tt class="class">DictMixin</tt>, defining all dictionary | |
| 65 | methods for classes that already have a minimum mapping interface. This | |
| 66 | greatly simplifies writing classes that need to be substitutable for | |
| 67 | dictionaries (such as the shelve module). | |
| 68 | ||
| 69 | <P> | |
| 70 | This also module defines a class, <tt class="class">UserDict</tt>, that acts as a wrapper | |
| 71 | around dictionary objects. The need for this class has been largely | |
| 72 | supplanted by the ability to subclass directly from <tt class="class">dict</tt> (a feature | |
| 73 | that became available starting with Python version 2.2). Prior to the | |
| 74 | introduction of <tt class="class">dict</tt>, the <tt class="class">UserDict</tt> class was used to | |
| 75 | create dictionary-like sub-classes that obtained new behaviors by overriding | |
| 76 | existing methods or adding new ones. | |
| 77 | ||
| 78 | <P> | |
| 79 | The <tt class="module">UserDict</tt> module defines the <tt class="class">UserDict</tt> class | |
| 80 | and <tt class="class">DictMixin</tt>: | |
| 81 | ||
| 82 | <P> | |
| 83 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 84 | <td><nobr><b><span class="typelabel">class</span> <tt id='l2h-478' xml:id='l2h-478' class="class">UserDict</tt></b>(</nobr></td> | |
| 85 | <td><var></var><big>[</big><var>initialdata</var><big>]</big><var></var>)</td></tr></table></dt> | |
| 86 | <dd> | |
| 87 | Class that simulates a dictionary. The instance's | |
| 88 | contents are kept in a regular dictionary, which is accessible via the | |
| 89 | <tt class="member">data</tt> attribute of <tt class="class">UserDict</tt> instances. If | |
| 90 | <var>initialdata</var> is provided, <tt class="member">data</tt> is initialized with its | |
| 91 | contents; note that a reference to <var>initialdata</var> will not be kept, | |
| 92 | allowing it be used for other purposes. <span class="note"><b class="label">Note:</b> | |
| 93 | For backward compatibility, | |
| 94 | instances of <tt class="class">UserDict</tt> are not iterable.</span> | |
| 95 | </dl> | |
| 96 | ||
| 97 | <P> | |
| 98 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 99 | <td><nobr><b><span class="typelabel">class</span> <tt id='l2h-479' xml:id='l2h-479' class="class">IterableUserDict</tt></b>(</nobr></td> | |
| 100 | <td><var></var><big>[</big><var>initialdata</var><big>]</big><var></var>)</td></tr></table></dt> | |
| 101 | <dd> | |
| 102 | Subclass of <tt class="class">UserDict</tt> that supports direct iteration (e.g. | |
| 103 | <code>for key in myDict</code>). | |
| 104 | </dl> | |
| 105 | ||
| 106 | <P> | |
| 107 | In addition to supporting the methods and operations of mappings (see | |
| 108 | section <A href="typesmapping.html#typesmapping">2.3.8</A>), <tt class="class">UserDict</tt> and | |
| 109 | <tt class="class">IterableUserDict</tt> instances provide the following attribute: | |
| 110 | ||
| 111 | <P> | |
| 112 | <dl><dt><b><tt id='l2h-480' xml:id='l2h-480' class="member">data</tt></b></dt> | |
| 113 | <dd> | |
| 114 | A real dictionary used to store the contents of the <tt class="class">UserDict</tt> | |
| 115 | class. | |
| 116 | </dl> | |
| 117 | ||
| 118 | <P> | |
| 119 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 120 | <td><nobr><b><span class="typelabel">class</span> <tt id='l2h-481' xml:id='l2h-481' class="class">DictMixin</tt></b>(</nobr></td> | |
| 121 | <td><var></var>)</td></tr></table></dt> | |
| 122 | <dd> | |
| 123 | Mixin defining all dictionary methods for classes that already have | |
| 124 | a minimum dictionary interface including <tt class="method">__getitem__()</tt>, | |
| 125 | <tt class="method">__setitem__()</tt>, <tt class="method">__delitem__()</tt>, and <tt class="method">keys()</tt>. | |
| 126 | ||
| 127 | <P> | |
| 128 | This mixin should be used as a superclass. Adding each of the | |
| 129 | above methods adds progressively more functionality. For instance, | |
| 130 | defining all but <tt class="method">__delitem__</tt> will preclude only <tt class="method">pop</tt> | |
| 131 | and <tt class="method">popitem</tt> from the full interface. | |
| 132 | ||
| 133 | <P> | |
| 134 | In addition to the four base methods, progressively more efficiency | |
| 135 | comes with defining <tt class="method">__contains__()</tt>, <tt class="method">__iter__()</tt>, and | |
| 136 | <tt class="method">iteritems()</tt>. | |
| 137 | ||
| 138 | <P> | |
| 139 | Since the mixin has no knowledge of the subclass constructor, it | |
| 140 | does not define <tt class="method">__init__()</tt> or <tt class="method">copy()</tt>. | |
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