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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 __prefix_key(self, key):
 """ This will add the prefix to the key if one exists on the store """

        # If there isn't a prefix don't bother
        if self.prefix is None:
            return key

        # Don't prefix key if it already has it
        if key.startswith(self.prefix + "-"):
            return key

        return "{0}-{1}".format(self.prefix, key)