sam-t2/devtools/amd64/html/python/lib/truth.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="boolean.html" />
<link rel="prev" href="types.html" />
<link rel="parent" href="types.html" />
<link rel="next" href="boolean.html" />
<meta name='aesop' content='information' />
<title>2.3.1 Truth Value Testing</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="2.3 Built-in Types"
  href="types.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="2.3 Built-in Types"
  href="types.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="2.3.2 Boolean Operations "
  href="boolean.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="types.html">2.3 Built-in Types</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="types.html">2.3 Built-in Types</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="boolean.html">2.3.2 Boolean Operations </A>
</div>
<hr /></div>
</DIV>
<!--End of Navigation Panel-->

<H2><A NAME="SECTION004310000000000000000"></A><A NAME="truth"></A>
<BR>
2.3.1 Truth Value Testing
</H2>

<P>
Any object can be tested for truth value, for use in an <tt class="keyword">if</tt> or
<tt class="keyword">while</tt> condition or as operand of the Boolean operations below.
The following values are considered false:
<a id='l2h-91' xml:id='l2h-91'></a><a id='l2h-92' xml:id='l2h-92'></a><a id='l2h-93' xml:id='l2h-93'></a><a id='l2h-94' xml:id='l2h-94'></a><a id='l2h-101' xml:id='l2h-101'></a>

<P>

<UL>
<LI><code>None</code>
        <a id='l2h-96' xml:id='l2h-96'></a>
<P>
</LI>
<LI><code>False</code>
        <a id='l2h-98' xml:id='l2h-98'></a>
<P>
</LI>
<LI>zero of any numeric type, for example, <code>0</code>, <code>0L</code>,
        <code>0.0</code>, <code>0j</code>.

<P>
</LI>
<LI>any empty sequence, for example, <code>''</code>, <code>()</code>, <code>[]</code>.

<P>
</LI>
<LI>any empty mapping, for example, <code>{}</code>.

<P>
</LI>
<LI>instances of user-defined classes, if the class defines a
        <tt class="method">__nonzero__()</tt> or <tt class="method">__len__()</tt> method, when that
        method returns the integer zero or <tt class="class">bool</tt> value
        <code>False</code>.<A NAME="tex2html6"
  HREF="#foot3425"><SUP>2.5</SUP></A>
<P>
</LI>
</UL>

<P>
All other values are considered true -- so objects of many types are
always true.
<a id='l2h-102' xml:id='l2h-102'></a>

<P>
Operations and built-in functions that have a Boolean result always
return <code>0</code> or <code>False</code> for false and <code>1</code> or <code>True</code>
for true, unless otherwise stated.  (Important exception: the Boolean
operations "<tt class="samp">or</tt>"<a id='l2h-99' xml:id='l2h-99'></a> and "<tt class="samp">and</tt>"<a id='l2h-100' xml:id='l2h-100'></a> always
return one of their operands.)
<a id='l2h-103' xml:id='l2h-103'></a>

<P>
<BR><HR><H4>Footnotes</H4>
<DL>
<DT><A NAME="foot3425">...False.</A><A
 href="truth.html#tex2html6"><SUP>2.5</SUP></A></DT>
<DD>Additional
information on these special methods may be found in the
<em class="citetitle"><a
 href="../ref/ref.html"
 title="Python Reference Manual"
 >Python Reference Manual</a></em>.

</DD>
</DL>
<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="2.3 Built-in Types"
  href="types.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="2.3 Built-in Types"
  href="types.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="2.3.2 Boolean Operations "
  href="boolean.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="types.html">2.3 Built-in Types</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="types.html">2.3 Built-in Types</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="boolean.html">2.3.2 Boolean Operations </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>