Question

I’m at the point in learning Python where I’m dealing with the Mutable Default Argument problem.

# BAD: if `a_list` is not passed in, the default will wrongly retain its contents between successive function calls
def bad_append(new_item, a_list=[]):
    a_list.append(new_item)
    return a_list

# GOOD: if `a_list` is not passed in, the default will always correctly be []
def good_append(new_item, a_list=None):
    if a_list is None:
        a_list = []
    a_list.append(new_item)
    return a_list

I understand that a_list is initialized only when the def statement is first encountered, and that’s why subsequent calls of bad_append use the same list object.

What I don’t understand is why good_append works any different. It looks like a_list would still be initialized only once; therefore, the if statement would only be true on the first invocation of the function, meaning a_list would only get reset to [] on the first invocation, meaning it would still accumulate all past new_item values and still be buggy.

Why isn’t it? What concept am I missing? How does a_list get wiped clean every time good_append runs?

Answer 1

The default value of a_list (or any other default value, for that matter) is stored in the function’s interiors once it has been initialized and thus can be modified in any way:

>>> def f(x=[]): return x
...
>>> f.func_defaults
([],)
>>> f.func_defaults[0] is f()
True

resp. for Python 3:

>>> def f(x=[]): return x
...
>>> f.__defaults__
([],)
>>> f.__defaults__[0] is f()
True

So the value in func_defaults is the same which is as well known inside function (and returned in my example in order to access it from outside.

In other words, what happens when calling f() is an implicit x = f.func_defaults[0]. If that object is modified subsequently, you’ll keep that modification.

In contrast, an assignment inside the function gets always a new []. Any modification will last until the last reference to that [] has gone; on the next function call, a new [] is created.

In order words again, it is not true that [] gets the same object on every execution, but it is (in the case of default argument) only executed once and then preserved.