If you feel like you have a legitimate reason to wash the snake, then check these things:
---Make sure the sink/tub/basin that you use is clean and free of chemical residue such as left-over soap or cleaners.
---Check water temps BEFORE getting the snake near the water. You want it to be room temperature or just a tiny bit warmer...so that when you touch the inside of your wrist to the water, it should feel neither hot nor cold. What might feel pleasantly "warm" to your hands could feel uncomfortably hot to a cold-blooded snake.
Legitimate reasons for bathing a ball python:
---They crawled through their poop and got it smeared all over them.
---They coiled around a f/t rat and made it explode rat guts and blood all over them. (Had it happen...NOT a pretty picture!)
---They failed to shed completely and have multiple patches of stuck shed that need a little help to loosen up.
There really isn't any reason to bath a ball python on any kind of regular or routine basis, and many will find it to be a stressful event.