Ensures that flags are not None during program execution.

If your module might be imported by others, and you only wish to make the flag required when the module is directly executed, call this method like this:

if name == 'main': flags.mark_flags_as_required(['flag1', 'flag2', 'flag3'])

flag_names Sequence[str], names of the flags.
flag_values flags.FlagValues, optional FlagValues instance where the flags are defined.

AttributeError If any of flag name has not already been defined as a flag.