<!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.gif" /> <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="prev" href="debugger-commands.html"> <LINK rel="parent" href="module-pdb.html"> <LINK rel="next" href="profile.html"> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> <meta name='aesop' content='information' /> <META name="description" content="How It Works "> <META name="keywords" content="lib"> <META name="resource-type" content="document"> <META name="distribution" content="global"> <title>9.2 How It Works </title> </head> <body> <DIV CLASS="navigation"> <div id='top-navigation-panel'> <table align="center" width="100%" cellpadding="0" cellspacing="2"> <tr> <td class='online-navigation'><a rel="prev" title="9.1 Debugger Commands" href="debugger-commands.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="9. 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The Python Profiler</A> </div> <hr /></div> </DIV> <!--End of Navigation Panel--> <H1><A NAME="SECTION0011200000000000000000"><!--x--></A><A NAME="debugger-hooks"><!--z--></A> <BR> 9.2 How It Works </H1> <P> Some changes were made to the interpreter: <P> <UL> <LI><code>sys.settrace(<var>func</var>)</code> sets the global trace function </LI> <LI>there can also a local trace function (see later) </LI> </UL> <P> Trace functions have three arguments: <var>frame</var>, <var>event</var>, and <var>arg</var>. <var>frame</var> is the current stack frame. <var>event</var> is a string: <code>'call'</code>, <code>'line'</code>, <code>'return'</code> or <code>'exception'</code>. <var>arg</var> depends on the event type. <P> The global trace function is invoked (with <var>event</var> set to <code>'call'</code>) whenever a new local scope is entered; it should return a reference to the local trace function to be used that scope, or <code>None</code> if the scope shouldn't be traced. <P> The local trace function should return a reference to itself (or to another function for further tracing in that scope), or <code>None</code> to turn off tracing in that scope. <P> Instance methods are accepted (and very useful!) as trace functions. <P> The events have the following meaning: <P> <DL> <DT><STRONG><code>'call'</code></STRONG></DT> <DD>A function is called (or some other code block entered). The global trace function is called; <var>arg</var> is <code>None</code>; the return value specifies the local trace function. <P> </DD> <DT><STRONG><code>'line'</code></STRONG></DT> <DD>The interpreter is about to execute a new line of code (sometimes multiple line events on one line exist). The local trace function is called; <var>arg</var> is <code>None</code>; the return value specifies the new local trace function. <P> </DD> <DT><STRONG><code>'return'</code></STRONG></DT> <DD>A function (or other code block) is about to return. The local trace function is called; <var>arg</var> is the value that will be returned. The trace function's return value is ignored. <P> </DD> <DT><STRONG><code>'exception'</code></STRONG></DT> <DD>An exception has occurred. The local trace function is called; <var>arg</var> is a triple <code>(<var>exception</var>, <var>value</var>, <var>traceback</var>)</code>; the return value specifies the new local trace function. <P> </DD> </DL> <P> Note that as an exception is propagated down the chain of callers, an <code>'exception'</code> event is generated at each level. <P> For more information on code and frame objects, refer to the <em class="citetitle"><a href="../ref/ref.html" title="Python Reference Manual" >Python Reference Manual</a></em>. <P> <DIV CLASS="navigation"> <div class='online-navigation'><hr /> <table align="center" width="100%" cellpadding="0" cellspacing="2"> <tr> <td class='online-navigation'><a rel="prev" title="9.1 Debugger Commands" rel="prev" title="9.1 Debugger Commands" href="debugger-commands.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="9. 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The Python Debugger</A> <b class="navlabel">Next:</b> <a class="sectref" rel="next" href="profile.html">10. The Python Profiler</A> </div> </div> <hr /> <span class="release-info">Release 2.3.4, documentation updated on May 20, 2004.</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>
