Default arguments are a helpful feature, but there is one situation where they can be surprisingly unhelpful. Using a mutable type (like a list or dictionary) as a default argument and then modifying that argument can lead to strange results. It's usually best to avoid using mutable default arguments: to see why, try the following code locally.
Consider this function which adds items to a todo list. Users can provide their own todo list, or add items to a default list:
def todo_list(new_task, base_list=['wake up']): base_list.append(new_task) return base_list
We can call the function like this:
>>> todo_list("check the mail") ['wake up', 'check the mail']
So if later on we call it again:
>>> todo_list("begin orbital transfer")
The result is actully:
['wake up', 'check the mail', 'begin orbital transfer']
The list object base_list is only created once: when the todo_list function is defined. Lists are mutable objects. This list object is used every time the function is called, it isn't redefined each time the function is called. Because todo_list appends an item to the list, base_list can get longer each time that todo_list is called.