I bought my first Python last night for trading, and put an A3 shield generator on it. I was worried about how well it'd stand up to my awful piloting and interdictions, so I did a quick trade run and some heat tests.
A3 shield on the Python seems to give better protection than the A4 shield gave my old T7. I bashed into various walls, toastered myself for a few seconds and landed miserably, but lost no shield rings. I was interdicted by an Eagle too, which normally would have taken a bar off my T7's shield before I escaped, but again the Python's shield was unaffected. The Python has a lower hull mass than the T7, so I guess that's the reason.
Also, in case it's helpful, I decided that the Python starts taking heat damage at ~135% heat.