code: def declare_key_flag(flag_name, flag_values=_flagvalues.FLAGS):
    
    if flag_name in _helpers.SPECIAL_FLAGS:
      # Take care of the special flags, e.g., --flagfile, --undefok.
      # These flags are defined in SPECIAL_FLAGS, and are treated
      # specially during flag parsing, taking precedence over the
      # user-defined flags.
      _internal_declare_key_flags([flag_name],
                                  flag_values=_helpers.SPECIAL_FLAGS,
                                  key_flag_values=flag_values)
      return
    try:
      _internal_declare_key_flags([flag_name], flag_values=flag_values)
    except KeyError:
      raise ValueError('Flag --%s is undefined. To set a flag as a key flag '
                       'first define it in Python.' % flag_name)
docs: Declares one flag as key to the current module.
  
    Key flags are flags that are deemed really important for a module.
    They are important when listing help messages; e.g., if the
    --helpshort command-line flag is used, then only the key flags of the
    main module are listed (instead of all flags, as in the case of
    --helpfull).
  
    Sample usage:
  
      flags.declare_key_flag('flag_1')
  
    Args:
      flag_name: str, the name of an already declared flag.
          (Redeclaring flags as key, including flags implicitly key
          because they were declared in this module, is a no-op.)
      flag_values: FlagValues, the FlagValues instance in which the flag will
          be declared as a key flag. This should almost never need to be
          overridden.
  
    Raises:
      ValueError: Raised if flag_name not defined as a Python flag.