sam-t2/devtools/amd64/html/python/lib/immutable-transforms.html

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<link rel="STYLESHEET" href="lib.css" type='text/css' />
<link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" />
<link rel='start' href='../index.html' title='Python Documentation Index' />
<link rel="first" href="lib.html" title='Python Library Reference' />
<link rel='contents' href='contents.html' title="Contents" />
<link rel='index' href='genindex.html' title='Index' />
<link rel='last' href='about.html' title='About this document...' />
<link rel='help' href='about.html' title='About this document...' />
<link rel="next" href="comparison-to-builtin-set.html" />
<link rel="prev" href="set-example.html" />
<link rel="parent" href="module-sets.html" />
<link rel="next" href="comparison-to-builtin-set.html" />
<meta name='aesop' content='information' />
<title>5.15.3 Protocol for automatic conversion to immutable </title>
</head>
<body>
<DIV CLASS="navigation">
<div id='top-navigation-panel' xml:id='top-navigation-panel'>
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="5.15.2 Example"
  href="set-example.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="5.15 sets  "
  href="module-sets.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="5.15.4 Comparison to the"
  href="comparison-to-builtin-set.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="set-example.html">5.15.2 Example</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="module-sets.html">5.15 sets  </A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="comparison-to-builtin-set.html">5.15.4 Comparison to the</A>
</div>
<hr /></div>
</DIV>
<!--End of Navigation Panel-->

<H2><A NAME="SECTION0071530000000000000000"></A><A NAME="immutable-transforms"></A>
<BR>
5.15.3 Protocol for automatic conversion to immutable

</H2>

<P>
Sets can only contain immutable elements.  For convenience, mutable
<tt class="class">Set</tt> objects are automatically copied to an <tt class="class">ImmutableSet</tt>
before being added as a set element.

<P>
The mechanism is to always add a hashable element, or if it is not
hashable, the element is checked to see if it has an
<tt class="method">__as_immutable__()</tt> method which returns an immutable equivalent.

<P>
Since <tt class="class">Set</tt> objects have a <tt class="method">__as_immutable__()</tt> method
returning an instance of <tt class="class">ImmutableSet</tt>, it is possible to
construct sets of sets.

<P>
A similar mechanism is needed by the <tt class="method">__contains__()</tt> and
<tt class="method">remove()</tt> methods which need to hash an element to check
for membership in a set.  Those methods check an element for hashability
and, if not, check for a <tt class="method">__as_temporarily_immutable__()</tt> method
which returns the element wrapped by a class that provides temporary
methods for <tt class="method">__hash__()</tt>, <tt class="method">__eq__()</tt>, and <tt class="method">__ne__()</tt>.

<P>
The alternate mechanism spares the need to build a separate copy of
the original mutable object.

<P>
<tt class="class">Set</tt> objects implement the <tt class="method">__as_temporarily_immutable__()</tt>
method which returns the <tt class="class">Set</tt> object wrapped by a new class
<tt class="class">_TemporarilyImmutableSet</tt>.

<P>
The two mechanisms for adding hashability are normally invisible to the
user; however, a conflict can arise in a multi-threaded environment
where one thread is updating a set while another has temporarily wrapped it
in <tt class="class">_TemporarilyImmutableSet</tt>.  In other words, sets of mutable sets
are not thread-safe.

<P>

<DIV CLASS="navigation">
<div class='online-navigation'>
<p></p><hr />
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="5.15.2 Example"
  href="set-example.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="5.15 sets  "
  href="module-sets.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="5.15.4 Comparison to the"
  href="comparison-to-builtin-set.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="set-example.html">5.15.2 Example</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="module-sets.html">5.15 sets  </A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="comparison-to-builtin-set.html">5.15.4 Comparison to the</A>
</div>
</div>
<hr />
<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
</DIV>
<!--End of Navigation Panel-->
<ADDRESS>
See <i><a href="about.html">About this document...</a></i> for information on suggesting changes.
</ADDRESS>
</BODY>
</HTML>