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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 as_list(self, key):
 """ A convenience method which fetches the specified value, guaranteeing that it is a list. [1] [1] [1] """

        result = self[key]
        if isinstance(result, (tuple, list)):
            return list(result)
        return [result]