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<H1><A NAME="SECTION0011200000000000000000">
9.2 How It Works</A>
</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; arg is the argument list to the function;
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; arg in None; 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; arg 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; arg is
a triple (exception, value, traceback); 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
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title='Python Reference Manual'
>Python Reference Manual</a></em>.
<P>
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