I am working on doing some digital filter work using Python and Numpy/Scipy.

I'm using scipy.signal.iirdesign to generate my filter coefficents, but it requires the filter passband coefficents in a format I am not familiar with

wp, ws : float

  Passband and stopband edge frequencies, normalized from 0 to 1 (1 corresponds 
      to pi radians / sample). 
  For example:
  Lowpass: wp = 0.2, ws = 0.3
  Highpass: wp = 0.3, ws = 0.2

(from here)

I'm not familiar with digital filters (I'm coming from a hardware design background). In an analog context, I would determine the desired slope and the 3db down point, and calculate component values from that.

In this context, how do I take a known sample rate, a desired corner frequency, and a desired rolloff, and calculate the wp, ws values from that?

(This might be more appropriate for math.stackexchange. I'm not sure)