class list_wrapper(object):
  def __init__(self, items):
    self._items = items

  def __add__(self, items):
    return list_wrapper(self._items + items)

  def __repr__(self):
    return repr(self._items)

def maybe_add_to_list(l, foo):
  l += foo

l1 = [1, 2, 3]
l2 = list_wrapper([1, 2, 3])

maybe_add_to_list(l1, [4, 5, 6])
maybe_add_to_list(l2, [4, 5, 6])

print l1
print l2

# % python test.py
# [1, 2, 3, 4, 5, 6]
# [1, 2, 3]