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. -- http://mail.python.org/mailman/listinfo/python-list
