How do I read IASI GDS L1C in Python?
The Basic ENVISAT Atmospheric Toolbox (BEAT) contains a component called the Common Data Access toolbox (CODA; also available on github).  BEAT and CODA are written in C and have bindings for Python (as well as Matlab, IDL, and a few other languages).  Among the long list of supported products is IASI L1C.  Apparently, BEAT is going to be replaced by HARP (github page), but HARP reportedly does not yet support IASI L1C.  BEAT, CODA, and HARP are all available under the GPL.

Although the BEAT website reports Python bindings are only available for Python 2.7, I have successfully used CODA with Python 3.5 (I did not try BEAT as I do not currently need the higher-level functionality it offers).