[Thamnophis]

I know that lots of people use aspen bedding for their snakes. But mostly this are mouse-eating snakes like Pantherophis, Elaphe, Pithuophis, etc.

I do not know if aspen bedding is hard or soft, but when it is hard (and sharp) I would not use it, because this can damage their esophagus, stomach or intestines.
It is unavoidable that some of the bedding gets in the snake with its food.
I can not judge if this is bad in the case of aspen because I have never used it (or held it in my hands).

I personally use woodchips that are used for rodents etc.
I like this stuff. It has never given me any problems. Only important condition is that it is free of dust.
Sometimes a little piece of the woodchips are accidentally eaten bij the snakes, but I have never seen any constipation.

I have also used potting soil and this also gave no problems when it was digested.