code: def tf2zpk(b, a):
      
      import scipy.signal
      z,p,k = scipy.signal.tf2zpk(b, a)
      return z,p,k
docs: Return zero, pole, gain (z,p,k) representation from a numerator,
      denominator representation of a linear filter.
  
      Convert zero-pole-gain filter parameters to transfer function form
  
      :param ndarray b:  numerator polynomial.
      :param ndarray a: numerator and denominator polynomials.
  
      :return:
          * z : ndarray        Zeros of the transfer function.
          * p : ndarray        Poles of the transfer function.
          * k : float        System gain.
  
      If some values of b are too close to 0, they are removed. In that case, a
      BadCoefficients warning is emitted.
  
      .. doctest::
  
          >>> import scipy.signal
          >>> from spectrum.transfer import tf2zpk
          >>> [b, a] = scipy.signal.butter(3.,.4)
          >>> z, p ,k = tf2zpk(b,a)
  
      .. seealso:: :func:`zpk2tf`
      .. note:: wrapper of scipy function tf2zpk