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<title>3.7 UserDict -- Class wrapper for dictionary objects
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<H1><A NAME=
"SECTION005700000000000000000">
3.7 <tt class=
"module">UserDict
</tt> --
Class wrapper for dictionary objects
</A>
<A NAME=
"module-UserDict"></A>
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).
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.
The
<tt class=
"module">UserDict
</tt> module defines the
<tt class=
"class">UserDict
</tt> class
and
<tt class=
"class">DictMixin
</tt>:
<dl><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline">
<td><nobr><b><span class=
"typelabel">class
</span> <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>
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><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline">
<td><nobr><b><span class=
"typelabel">class
</span> <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>
Subclass of
<tt class=
"class">UserDict
</tt> that supports direct iteration (e.g.
<code>for key in myDict
</code>).
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:
<dl><dt><b><tt id='l2h-
480' xml:id='l2h-
480'
class=
"member">data
</tt></b></dt>
A real dictionary used to store the contents of the
<tt class=
"class">UserDict
</tt>
<dl><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline">
<td><nobr><b><span class=
"typelabel">class
</span> <tt id='l2h-
481' xml:id='l2h-
481'
class=
"class">DictMixin
</tt></b>(
</nobr></td>
<td><var></var>)
</td></tr></table></dt>
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>.
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.
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>.
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>.
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2005.
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