Re: bad recursion, still works

[oj]

On Jul 16, 1:09*pm, Jeff <[e-mail]:

On Jul 15, 7:21*pm, Michael Torrie <[e-mail]:

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.