code: def parse_column_key(setting_string):
          
          if ':' in setting_string:
              # splits <column>:<key> into <column> and <key>
              column, key = setting_string.split(':', 1)
          else:
              # stores <column> and <value>=None
              column = setting_string
              key = None
  
          return column, key
docs: Parses 'setting_string' as str formatted in <column>[:<key>]
          and returns str type 'column' and 'key'