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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 get_uri(self, alias):
 """ Get the URI for a given alias. A registered URI will return itself, otherwise ``None`` is returned. """

        if alias in self.aliases.keys():
            return self.aliases[alias]
        if alias in self.aliases.values():
            return alias
        raise GraphException('No such schema: %r' % alias)