flatten

# http://rightfootin.blogspot.com/2006/09/more-on-python-flatten.html

def flatten(l, ltypes=(list, tuple)):
  ltype, l = type(l), list(l)
  i = 0
  while i < len(l):
    while isinstance(l[i], ltypes):
      if not l[i]:
        l.pop(i)
        i -= 1
        break
      else:
        l[i:i + 1] = l[i]
    i += 1
  return ltype(l)