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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 refresh(self) -> None: """ Reloads all sources that are registered with this server. """

        logger.info('refreshing sources')
        for source in list(self):
            self.unload(source)

        if not os.path.exists(self.__registry_fn):
            return

        # TODO add version
        with open(self.__registry_fn, 'r') as f:
            registry = yaml.load(f)
        assert isinstance(registry, list)

        for source_description in registry:
            source = Source.from_dict(source_description)
            self.load(source)

        logger.info('refreshed sources')