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(python2.1-lib.info)How It Works

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How It Works
============

Some changes were made to the interpreter:

   * `sys.settrace(FUNC)' sets the global trace function

   * there can also a local trace function (see later)

Trace functions have three arguments: FRAME, EVENT, and ARG. FRAME is
the current stack frame.  EVENT is a string: `'call'', `'line'',
`'return'' or `'exception''.  ARG depends on the event type.

The global trace function is invoked (with EVENT set to `'call'')
whenever a new local scope is entered; it should return a reference to
the local trace function to be used that scope, or `None' 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 `None' to turn
off tracing in that scope.

Instance methods are accepted (and very useful!) as trace functions.

The events have the following meaning:

``'call'''
     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.

``'line'''
     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.

``'return'''
     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.

``'exception'''
     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

Note that as an exception is propagated down the chain of callers, an
`'exception'' event is generated at each level.

For more information on code and frame objects, refer to the .