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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 _list_keys(self):
 """ Retrieves a list of all added Keys and populates the self._keys dict with Key instances :returns: A list of Keys instances """

        req = self.request(self.uri + '/keys')
        keys = req.get().json()
        if keys:
            self._keys = {}
            for key in keys:
                self._keys[key['id']] = Key(key, self)
        else:
            self._keys = {}