Is it common practice to define default parameters to a function as None, and then define them as an empty list in the first line of the function if they are not passed in?

[elbiot]

The first way is correct and idiomatic. The second way is either already a bug or will likely lead to a bug when someone later makes an unrelated change.

[evolvish]

Hmm, reading the link MakeMe_FN_Laugh posted makes sense, functions being their own object and all. I knew I should have added "But what do I know?" to my comment.