[Python] Problem with Default Arguments

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.