def space(s, n):
  """inserts a space every n characters in string s
  
  >>> space('0123456789', 3)
  '012 345 678 9'

  >>> space('0123456789', 4)
  '0123 4567 89'
  """
  return ' '.join(s[i:i+n] for i in range(0, len(s), n))