code: def has_value(self, subsystem, option):
          
          assert subsystem in self
          return os.path.isfile(os.path.join(self.per_subsystem[subsystem], subsystem + '.' + option))
docs: Check whether the given value exists in the given subsystem.
          Does not make a difference whether the value is readable, writable, or both.
          Do not include the subsystem name in the option name.
          Only call this method if the given subsystem is available.