def get_control_outputs(self, op):
    """Return the control outputs for a given op.

    Args:
      op: The op to fetch control outputs for.

    Returns:
      Iterable of control output ops.
    """
    if op.graph not in self.cache:
      control_outputs = self.calc_control_outputs(op.graph)
      self.cache[op.graph] = control_outputs
    else:
      control_outputs = self.cache[op.graph]
    return control_outputs.get(op, [])