code: def flags_into_string(self):
      
      module_flags = sorted(self.flags_by_module_dict().items())
      s = ''
      for unused_module_name, flags in module_flags:
        flags = sorted(flags, key=lambda f: f.name)
        for flag in flags:
          if flag.value is not None:
            s += flag.serialize() + '\n'
      return s
docs: Returns a string with the flags assignments from this FlagValues object.
  
      This function ignores flags whose value is None.  Each flag
      assignment is separated by a newline.
  
      NOTE: MUST mirror the behavior of the C++ CommandlineFlagsIntoString
      from https://github.com/gflags/gflags.
  
      Returns:
        str, the string with the flags assignments from this FlagValues object.
        The flags are ordered by (module_name, flag_name).