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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_key(self, fileobj):
 """ Return the key associated with a registered file object. """

        mapping = self.get_map()
        if mapping is None:
            raise RuntimeError("Selector is closed")
        try:
            return mapping[fileobj]
        except KeyError:
            raise KeyError("{0!r} is not registered".format(fileobj))