I'm at the point in learning Python where I'm dealing with the Mutable Default Argument problem.
def bad_append(new_item, a_list=): a_list.append(new_item) return a_list 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
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 is f()
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.
IOW, what happens when calling
f() is an implicit
x = f.func_defaults. 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.
IOW 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.