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<H1><A NAME="SECTION005900000000000000000">
3.9 <tt class="module">UserString</tt> --
         Class wrapper for string objects</A>
</H1>

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

<P>
<span class="note"><b class="label">Note:</b>
This <tt class="class">UserString</tt> class from this module is available for
backward compatibility only.  If you are writing code that does not
need to work with versions of Python earlier than Python 2.2, please
consider subclassing directly from the built-in <tt class="class">str</tt> type
instead of using <tt class="class">UserString</tt> (there is no built-in equivalent
to <tt class="class">MutableString</tt>).</span>

<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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-486' xml:id='l2h-486' class="class">UserString</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>sequence</var><big>]</big><var></var>)</td></tr></table></dt>
<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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-487' xml:id='l2h-487' class="class">MutableString</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>sequence</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
This class is derived from the <tt class="class">UserString</tt> above and redefines
strings to be <em>mutable</em>.  Mutable strings can't be used as
dictionary keys, because dictionaries require <em>immutable</em> 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">2.3.6</A>, ``String
Methods''), <tt class="class">UserString</tt> instances provide the following
attribute:

<P>
<dl><dt><b><tt id='l2h-488' xml:id='l2h-488' class="member">data</tt></b></dt>
<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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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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