Registers a 'Flag' object with a 'FlagValues' object.

  By default, the global FLAGS 'FlagValue' object is used.

  Typical users will use one of the more specialized DEFINE_xxx
  functions, such as DEFINE_string or DEFINE_integer.  But developers
  who need to create Flag objects themselves should use this function
  to register their flags.

  Args:
    flag: Flag, a flag that is key to the module.
    flag_values: FlagValues, the FlagValues instance with which the flag will
        be registered. This should almost never need to be overridden.
    module_name: str, the name of the Python module declaring this flag.
        If not provided, it will be computed using the stack trace of this call.

def DEFINE_flag(flag, flag_values=_flagvalues.FLAGS, module_name=None):  # pylint: disable=invalid-name
  """Registers a 'Flag' object with a 'FlagValues' object.

  By default, the global FLAGS 'FlagValue' object is used.

  Typical users will use one of the more specialized DEFINE_xxx
  functions, such as DEFINE_string or DEFINE_integer.  But developers
  who need to create Flag objects themselves should use this function
  to register their flags.

  Args:
    flag: Flag, a flag that is key to the module.
    flag_values: FlagValues, the FlagValues instance with which the flag will
        be registered. This should almost never need to be overridden.
    module_name: str, the name of the Python module declaring this flag.
        If not provided, it will be computed using the stack trace of this call.
  """
  # Copying the reference to flag_values prevents pychecker warnings.
  fv = flag_values
  fv[flag.name] = flag
  # Tell flag_values who's defining the flag.
  if module_name:
    module = sys.modules.get(module_name)
  else:
    module, module_name = _helpers.get_calling_module_object_and_name()
  flag_values.register_flag_by_module(module_name, flag)
  flag_values.register_flag_by_module_id(id(module), flag)