Mutable Default Arguments#

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Seemingly the most common surprise new Python programmers encounter is Python’s treatment of mutable default arguments in function definitions.

What You Wrote

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

What You Might Have Expected to Happen

my_list = append_to(12)

my_other_list = append_to(42)

A new list is created each time the function is called if a second argument isn’t provided, so that the output is:


What Actually Happens

[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, not 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 Instead

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 = []
    return to

Do not forget, you are passing a list object as the second argument.

When the Gotcha Isn’t a Gotcha

Sometimes you can specifically “exploit” (read: use as intended) this behavior to maintain state between calls of a function. This is often done when writing a caching function.