In a Python Reactor, a Photonic Core converts photon particles into Python particles. A Python Inverter is essentially a python reactor without a photonic core.
The inverter still requires a light source, usually high-energy plasma inertial confined. Photons are then directed through the traps to seperate out the pythons. The key difference is that, with no photonic core, the excess photons are are simply directed away from the ship.
The power requirements for a python inverter drive are an order of magnitude higher than a python reactor. Some of this can be mitigated by using a constant matter/antimatter reaction for photon/python production, but the drive is severely limited in both ship mass, speed, and operational lifespan.
The highest recorded speed achieved by an Python Inverter-driven ship is FTL Factor 5.
Python Inverter technology is a very important stepping stone on the road to FTL technology. While the Photonic Core was developed independently by a number of civilizations (though Dr. Rupert Python still gets credit), all of those civilizations had to first perfect inverter drives. Countless other worlds developed inverters and used them to make contact with races that had more advanced technology.
While Python Inverters have been completely superseded by reactors, the operating principles still see highly limited use. In the event of damage to the photonic core, a modern python reactor-driven ship can, with minor modifications, use the reactor like an inverter for an emergency jump to a populated system.
Inverters have also seen use on relativistic velocity missiles (a favorite M'KHaren weapon). In this application, a very small, very simple python inverter is used in conjunction with a fusion rocket motor to propel a warhead up to 99 PSL. The missile actually breifly crosses the stop-light barrier, achieving a very low FTL factor, and then drops back down to 99 PSL when the drive fails(usually in under a 10th of a second). The missile actually has no explosive warhead, it just hits at relativistic velocity and uses it's own mass as a weapon. The Foundation experimented with the technology during the Kamian Succession Wars but found them too challenging to properly aim. Inteligence reports show the Kamians were working on developing a similar technology near the end of the war. The M'KHarens, who had a fully combat-ready model when they entered the conflict, were able to use them to great effect.