I always thought that those stories about critters coming up out of the toilet (usually when people come home after a longish trip) were urban myths. I mean, the pipes are perpendicular and there's nothing to grab, so how could they climb up in the first place, and surely they would drown. I'm never again going to sit down without looking first.
Even in a perpendicular pipe, a reasonably-sized snake coils in the pipe, adjusts so that it's firmly against both sides of the pipe, and slithers up, using it's own muscles to provide traction. AFAIK, the only part of the pipe that's full of water is the one sharp bend that uses water to stop sewer gas from backing up. Snake holds it's breath, and, well, 'dives'. His head probably sees daylight before it's midsection gets through.
I would think that if the snake sees the 'light at the end of the tunnel' suddenly go dark, it would wait and see if things change before making that final attempt.