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<title>9.2 How It Works
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<H1><A NAME=
"SECTION0011200000000000000000"></A><A NAME=
"debugger-hooks"></A>
Some changes were made to the interpreter:
<LI><code>sys.settrace(
<var>func
</var>)
</code> sets the global trace function
<LI>there can also a local trace function (see later)
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>,
<code>'exception'
</code>,
<code>'c_call'
</code>,
<code>'c_return'
</code>, or
<code>'c_exception'
</code>.
<var>arg
</var>
depends on the event type.
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.
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.
Instance methods are accepted (and very useful!) as trace functions.
The events have the following meaning:
<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.
<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
<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.
<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
<DT><STRONG><code>'c_call'
</code></STRONG></DT>
<DD>A C function is about to be called. This may be an extension function
or a builtin.
<var>arg
</var> is the C function object.
<DT><STRONG><code>'c_return'
</code></STRONG></DT>
<DD>A C function has returned.
<var>arg
</var> is
<code>None
</code>.
<DT><STRONG><code>'c_exception'
</code></STRONG></DT>
<DD>A C function has thrown an exception.
<var>arg
</var> is
<code>None
</code>.
Note that as an exception is propagated down the chain of callers, an
<code>'exception'
</code> event is generated at each level.
For more information on code and frame objects, refer to the
title=
"Python Reference Manual"
>Python Reference Manual
</a></em>.
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<span class=
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2.4.2, documentation updated on
28 September
2005.
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