code: def parse(handle):
      
  
      components = handle.split('/', 1)
      if len(components) == 1:
          return None, components[0]
      elif len(components) == 2:
          return components[0], components[1]
      else:
          raise BinstarError("{} can't be parsed".format(handle))
docs: >>> parse("user/notebook")
      ('user', 'notebook')
      >>> parse("notebook")
      (None, 'notebook')
  
      :param handle: String
      :return: username, notebooks