code: def get_value(self, subsystem, option):
          
          assert subsystem in self, 'Subsystem {} is missing'.format(subsystem)
          return util.read_file(self.per_subsystem[subsystem], subsystem + '.' + option)
docs: Read the given value from the given subsystem.
          Do not include the subsystem name in the option name.
          Only call this method if the given subsystem is available.