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<H1><A NAME="SECTION005700000000000000000">
3.7 <tt class="module">UserDict</tt> --
         Class wrapper for dictionary objects</A>
</H1>

<P>
<A NAME="module-UserDict"></A>

<P>
The module defines a mixin,  <tt class="class">DictMixin</tt>, defining all dictionary
methods for classes that already have a minimum mapping interface.  This
greatly simplifies writing classes that need to be substitutable for
dictionaries (such as the shelve module).

<P>
This also module defines a class, <tt class="class">UserDict</tt>, that acts as a wrapper
around dictionary objects.  The need for this class has been largely
supplanted by the ability to subclass directly from <tt class="class">dict</tt> (a feature
that became available starting with Python version 2.2).  Prior to the
introduction of <tt class="class">dict</tt>, the <tt class="class">UserDict</tt> class was used to
create dictionary-like sub-classes that obtained new behaviors by overriding
existing methods or adding new ones.

<P>
The <tt class="module">UserDict</tt> module defines the <tt class="class">UserDict</tt> class
and <tt class="class">DictMixin</tt>:

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-478' xml:id='l2h-478' class="class">UserDict</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>initialdata</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
Class that simulates a dictionary.  The instance's
contents are kept in a regular dictionary, which is accessible via the
<tt class="member">data</tt> attribute of <tt class="class">UserDict</tt> instances.  If
<var>initialdata</var> is provided, <tt class="member">data</tt> is initialized with its
contents; note that a reference to <var>initialdata</var> will not be kept,
allowing it be used for other purposes. <span class="note"><b class="label">Note:</b>
For backward compatibility,
instances of <tt class="class">UserDict</tt> are not iterable.</span>
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-479' xml:id='l2h-479' class="class">IterableUserDict</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>initialdata</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
Subclass of <tt class="class">UserDict</tt> that supports direct iteration (e.g.
<code>for key in myDict</code>).
</dl>

<P>
In addition to supporting the methods and operations of mappings (see
section <A href="typesmapping.html#typesmapping">2.3.8</A>), <tt class="class">UserDict</tt> and
<tt class="class">IterableUserDict</tt> instances provide the following attribute:

<P>
<dl><dt><b><tt id='l2h-480' xml:id='l2h-480' class="member">data</tt></b></dt>
<dd>
A real dictionary used to store the contents of the <tt class="class">UserDict</tt>
class.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-481' xml:id='l2h-481' class="class">DictMixin</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Mixin defining all dictionary methods for classes that already have
a minimum dictionary interface including <tt class="method">__getitem__()</tt>,
<tt class="method">__setitem__()</tt>, <tt class="method">__delitem__()</tt>, and <tt class="method">keys()</tt>.

<P>
This mixin should be used as a superclass.  Adding each of the
above methods adds progressively more functionality.  For instance,
defining all but <tt class="method">__delitem__</tt> will preclude only <tt class="method">pop</tt>
and <tt class="method">popitem</tt> from the full interface.

<P>
In addition to the four base methods, progressively more efficiency
comes with defining <tt class="method">__contains__()</tt>, <tt class="method">__iter__()</tt>, and
<tt class="method">iteritems()</tt>.

<P>
Since the mixin has no knowledge of the subclass constructor, it
does not define <tt class="method">__init__()</tt> or <tt class="method">copy()</tt>.
</dl>

<P>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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