Not trying to nitpick, but since when is 73 at all ok for a ball python? I'll admit I've only really been keeping them for a couple years now but I've always understood the minimum ambients to be 80°. I'm sure it'd be fine with an occasional dip to 78, but I would've thought that any significant time at 73 would result in an RI of some sort.
In the winters my night temps in the basement where I keep mine get into upper 60s, for months at a time. Ambient temps year round for me are pretty much 73-78 like Mr Lang said.