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<H1><A NAME="SECTION005700000000000000000">
3.7 <tt class="module">UserString</tt> --
Class wrapper for string objects</A>
</H1>
<P>
<P>
This module defines a class that acts as a wrapper around string
objects. It is a useful base class for your own string-like classes,
which can inherit from them and override existing methods or add new
ones. In this way one can add new behaviors to strings.
<P>
It should be noted that these classes are highly inefficient compared
to real string or Unicode objects; this is especially the case for
<tt class="class">MutableString</tt>.
<P>
The <tt class="module">UserString</tt> module defines the following classes:
<P>
<dl><dt><b><a name='l2h-323'><tt class='class'>UserString</tt></a></b> (<big>[</big><var>sequence</var><big>]</big>)
<dd>
Class that simulates a string or a Unicode string
object. The instance's content is kept in a regular string or Unicode
string object, which is accessible via the <tt class="member">data</tt> attribute of
<tt class="class">UserString</tt> instances. The instance's contents are initially
set to a copy of <var>sequence</var>. <var>sequence</var> can be either a
regular Python string or Unicode string, an instance of
<tt class="class">UserString</tt> (or a subclass) or an arbitrary sequence which can
be converted into a string using the built-in <tt class="function">str()</tt> function.
</dl>
<P>
<dl><dt><b><a name='l2h-324'><tt class='class'>MutableString</tt></a></b> (<big>[</big><var>sequence</var><big>]</big>)
<dd>
This class is derived from the <tt class="class">UserString</tt> above and redefines
strings to be <i>mutable</i>. Mutable strings can't be used as
dictionary keys, because dictionaries require <i>immutable</i> objects as
keys. The main intention of this class is to serve as an educational
example for inheritance and necessity to remove (override) the
<tt class="method">__hash__()</tt> method in order to trap attempts to use a
mutable object as dictionary key, which would be otherwise very
error prone and hard to track down.
</dl>
<P>
In addition to supporting the methods and operations of string and
Unicode objects (see section <A href="string-methods.html#string-methods" tppabs="http://www.python.org/doc/current/lib/string-methods.html#string-methods">2.1.5</A>, ``String
Methods''), <tt class="class">UserString</tt> instances provide the following
attribute:
<P>
<dl><dt><b><a name='l2h-325'><tt class='member'>data</tt></a></b>
<dd>
A real Python string or Unicode object used to store the content of the
<tt class="class">UserString</tt> class.
</dl>
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