code: def _warning(code):
      
      if isinstance(code, str):
          return code
  
      message = ''
      if isinstance(code, tuple):
          if isinstance(code[0], str):
              message = code[1]
              code = code[0]
      return CFG_BIBRECORD_WARNING_MSGS.get(code, '') + message
docs: Return a warning message of code 'code'.
  
      If code = (cd, str) it returns the warning message of code 'cd' and appends
      str at the end