How do I apply a Chebishev filter?
The main issue with the example you gave is that the filter design function cheby1 is returning all NaNs, which isn't going to be a very good filter. The problem is how you're specifying the passband/stopband edge frequencies. This particular function is meant to emulate MATLAB's cheby1 function; the frequencies that you give it should be normalized, such that a value of 1 corresponds to half of the sample rate.

import scipy.signal as signal
fs = whatever_the_sample_rate_of_the_filter_input_is_going_to_be
signal.cheby1(8,0.05,[0.1/(fs/2),10.0/(fs/2)],btype='band',analog=0,output='ba')


I don't have SciPy handy, but that should at least correctly design the filter you want.