code: def is_none(string_, default='raise'):
      
      none = ['none', 'undefined', 'unknown', 'null', '']
      if string_.lower() in none:
          return True
      elif not default:
          return False
      else:
          raise ValueError('The value \'{}\' cannot be mapped to none.'
                           .format(string_))
docs: Check if a string is equivalent to None.
  
      Parameters
      ----------
      string_ : str
      default : {'raise', False}
          Default behaviour if none of the "None" strings is detected.
  
      Returns
      -------
      is_none : bool
  
      Examples
      --------
      >>> is_none('2', default=False)
      False
      >>> is_none('undefined', default=False)
      True