code: def append_flags_into_file(self, filename):
      
      with open(filename, 'a') as out_file:
        out_file.write(self.flags_into_string())
docs: Appends all flags assignments from this FlagInfo object to a file.
  
      Output will be in the format of a flagfile.
  
      NOTE: MUST mirror the behavior of the C++ AppendFlagsIntoFile
      from https://github.com/gflags/gflags.
  
      Args:
        filename: str, name of the file.