Hello, I am professional prototype engineer with 6 years experience in big automotive company. What you need to do to reverse the stepper motor is to drive it's four pins in the opposite steps that are now. Here it is:
straight:phase_seq=[[1,0,0,0],[1,1,0,0],[0,1,0,0],[0,1,1,0],[0,0,1,0],[0,0,1,1],[0,0,0,1],[1,0,0,1]]
in the revers function, just replace this sequence in the reverse order and the motor will rotate backwards
Looking forward to working with you!