As it stands, a function that takes zeroes, poles, gain into numerator, denominator transfer function coefficient arrays only lives in scipy.signal or can be imported from control.matlab, and used to create a transfer function my_tf as follows:
my_tf= tf(*zpk2tf(...))
Why not instead can python-control have a method that directly takes zeroes, poles, gain into a transfer function, i.e.
my_tf=zpk2tf(..)?