bad recursion, still works

[oj]

On Jul 16, 1:09 pm, Jeff <jeffo... at gmail.com> wrote:
> On Jul 15, 7:21 pm, Michael Torrie <torr... at gmail.com> wrote:
>
> > iu2 wrote:
> > > I still don't understand: In each recursive call to flatten, acc
> > > should be bound to a new [], shouldn't it? Why does the binding happen
> > > only on the first call to flatten?
>
> > Nope.  In each new call it's (re)bound to the same original list, which
> > you've added to as your function continues--it's mutable.  Default
> > variables that are bound to mutable objects are one of the big caveats
> > that is mentioned in the FAQ.
>
> Is this avoidable by using a call to list() in the definition instead?

No.

Probably what you'd want to do, is something like this:

def func(arg1, arg2=None):
    if arg2 is None:
        arg2 = list()

    ...

So you create a list at runtime if arg2 has its default value.