How to find the group delay of IIR filters in Python/SciPy
I only need the total group delay, not spectrum of group delay.


Group delay is a spectrum, so this doesn't make sense.  The group delay is the derivative of the phase response of the filter, so in Python it can be calculated as 

from scipy import signal
from numpy import pi, diff, unwrap, angle

w, h = signal.freqs(b, a)
group_delay = -diff(unwrap(angle(h))) / diff(w)


The units of phase are radians, and the units of frequency are radians/sec, so the units of group delay are in seconds.  Examples at 1 and 2

The group delay of a Bessel filter is maximally flat, so when you say "the" group delay, you probably mean the maximum passband group delay.  Like in this filter:



The passband has about 0.032 seconds of group delay.  Your Butterworth filter does not attempt to have a flat group delay in the passband, however, so it's up to you to decide what "the" group delay is:



Do you want the group delay at 0 Hz, or the maximum group delay at the cutoff frequency?  Or maybe at a specific frequency like the fundamental clock frequency of your digital signal?