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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 check_expired_activation(self, activation_key):
 """ Check if ``activation_key`` is still valid. Raises a ``self.model.DoesNotExist`` exception if key is not present or ``activation_key`` is not a valid string :param activation_key: String containing the secret SHA1 for a valid activation. :return: True if the ket has expired, False if still valid. """

        if SHA1_RE.search(activation_key):
            userena = self.get(activation_key=activation_key)
            return userena.activation_key_expired()
        raise self.model.DoesNotExist