Great library!

In signal.weigh there is the zero-phase option that uses scipy.signal.filtfilt.

To inspect the frequency response of the zero-phase (forwards-backwards operation) filter Hzp, it is necessary to calculate the frequency response of the forwards filter H (using scipy.signal.freqz), and then multiply it by the complex conjugate H*. Doing this shows that the forwards-backwards operation (ie applying the filter two times to the signal) distorts the overall frequency response Hzp, which in many cases will not give the desired response. In this case, zero-phase operation pushes the filter response outside the relevant IEC acceptance limits.

To avoid this distortion it would be necessary to pre-warp the filter frequencies (in a similar way to compensating for bilinear transform distortion), although I'm unaware of any way to determine the appropriate pre-warping analytically for arbitrary filter responses.