The function doc:

python-control/control/timeresp.py

Line 774 in 92de12d

Undershoot: Percentage of undershoot

the code calculate undershoot as:

python-control/control/timeresp.py

Line 818 in 92de12d

'Undershoot': yout.min(), # not very confident about this

An example:

import numpy as np              # NumPy library
import matplotlib.pyplot as plt
import control as ctrl  # Name: control Version: 0.8.4

sys3 = ctrl.tf([-1, 1],[1,1,1])
ctrl.step_info(sys3)
time,yout=ctrl.step_response(sys3)
plt.figure()
plt.plot(time,yout)

{'RiseTime': 1.197344248356904,
'SettlingTime': 6.631445067822853,
'SettlingMin': 0.8878785981432115,
'SettlingMax': 1.2087112656708903,
'Overshoot': 23.04582827419207,
'Undershoot': -0.2791703847438639,
'Peak': 1.2087112656708903,
'PeakTime': 4.236756571109045,
'SteadyStateValue': 0.9823260833983173}