[Python] Problem with Default Arguments

Default arguments is a helpful feature and there is one situation where they can surprisingly unhelpful. Using a mutable type (like a list or dictionary) as a default argument and then modifying that argument can leads to Strang E results. It's usually best to avoid using mutable default arguments:to see why, try the following code locally.

Consider this function which the 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=['wakeup'):    base_list.append (new_ Task)    return base_list

We can call the function as this:

>>> todo_list ("check the mail") ['wake up ' ' Check the Mail ']

If later on the We call it again:

>>> todo_list ("begin orbital Transfer")

The result is actully:

['wakeup'Check the mail'begin Orbital Transfer']

The list object is a base_list created once: when the todo_list function is defined. Lists is mutable objects. This list object was 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 this is todo_list called.

[Python] Problem with Default Arguments