In the first example under "When to use which", the reason you want to set a default argument in a function to None, check if it is None, and then set it to an

In the first example under "When to use which", the reason you want to set a default argument in a function to None, check if it is None, and then set it to an empty list is explained here: http://effbot.org/zone/default-values.htm http://effbot.org/zone/default-values.htm In short, it can be bad to set default arguments to mutable objects because the function keeps using the same object in each call.