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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 clean_cache_upstream(self):
 """Clean cache for all steps that are upstream to `self`. """

        logger.info('Cleaning cache for the entire upstream pipeline')
        for step in self.all_upstream_steps.values():
            logger.info('Step {}, cleaning cache'.format(step.name))
            step.output = None
        return self