recursive dict comparison with self-reference

dictcmp.py

def dictcmp(a, b, stacka=None, stackb=None):
  if a is b:
    return True
  if type(a) is not type(b):
    return False
  stacka = stacka or []
  stackb = stackb or []
  stacka.append(id(a))
  stackb.append(id(b))
  res = False
  if isinstance(a, (list, dict)):
    ia = stacka.index(id(a))
    ib = stackb.index(id(b))
    if ia != ib:
      res = False
    elif ia < len(stacka) - 1:
      res = True # already comparing these
    else:
      if isinstance(a, list):
        res = all(dictcmp(ia, ib, stacka, stackb) for ia, ib in zip(a, b))
      elif isinstance(a, dict):
        ka = a.keys()
        if ka != b.keys():
          res = False
        else:
          res = all(dictcmp(a[k], b[k], stacka, stackb) for k in ka)
  else:
    res = a == b
 
  stacka.pop()
  stackb.pop()
  return res