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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 backend_to_retrieve(self, namespace, stream):
 """ Return backend enabled for reading for `stream`. """

    if namespace not in self.namespaces:
      raise NamespaceMissing('`{}` namespace is not configured'
                             .format(namespace))
    stream_prefix = self.get_matching_prefix(namespace, stream)
    read_backend = self.prefix_read_backends[namespace][stream_prefix]
    return (read_backend,
            self.prefix_confs[namespace][stream_prefix][read_backend])