def setDebugActions( self, startAction, successAction, exceptionAction ):
        """
        Enable display of debugging messages while doing pattern matching.
        """
        self.debugActions = (startAction or _defaultStartDebugAction,
                             successAction or _defaultSuccessDebugAction,
                             exceptionAction or _defaultExceptionDebugAction)
        self.debug = True
        return self

        Enable display of debugging messages while doing pattern matching.
        
src/pkg_resources/_vendor/pyparsing.py