def fetch_keys(self, filter):
        """Yields one or more keys from the key vault.

        :Parameters:
          - `filter`: The filter to pass to find.

        :Returns:
          A generator which yields the requested keys from the key vault.
        """
        with self.key_vault_coll.find(RawBSONDocument(filter)) as cursor:
            for key in cursor:
                yield key.raw
Yields one or more keys from the key vault.

        :Parameters:
          - `filter`: The filter to pass to find.

        :Returns:
          A generator which yields the requested keys from the key vault.
        
stackoverflow/venv/lib/python3.6/site-packages/pymongo/encryption.py