Python: Mutable default arguments

One of the most confusing moments for new developers is when they discover how Python treats default arguments in function definitions.

Let’s say you want to define a function that accepts a list as a parameter, and you want the default value of that list to be the empty list, so you write:

def append_to(element, to=[]):
    to.append(element)
    return to

If everything was okay, you would expect the following behavior:

my_list = append_to(12)
print my_list

my_other_list = append_to(42)
print my_other_list

[12]
[42]

But what you get instead is:

[12]
[12, 42]

A new list is created once when the function is defined, and the same list is used in each successive call.

Python’s default arguments are evaluated once when the function is defined, instead of each time the function is called (like it is in say Ruby). This means that if you use a mutable default argument and mutate it, you will and have mutated that object for all future calls to the function as well.

What you should do

Create a new object each time the function is called, by using a default arg to signal that no argument was provided (None is often a good choice):

def append_to(element, to=None):
    if to is None:
        to = []
    to.append(element)
    return to

Via thoughtbot/til and python guide .