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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def progress_callback(self, action, node, elapsed_time=None):
 """ Callback to report progress :param str action: :param list node: app, module :param int | None elapsed_time: """

        if action == 'load_start':
            self.stdout.write('Loading fixture {}.{}...'.format(*node),
                              ending='')
            self.stdout.flush()
        elif action == 'load_success':
            message = 'SUCCESS'
            if elapsed_time:
                message += ' ({:.03} seconds) '.format(elapsed_time)

            self.stdout.write(message)