def open(self, srv, req):
        if not self.files.has_key(req.fid.qid.path):
            srv.respond(req, "unknown file")
        f = self.files[req.fid.qid.path]
        if (req.ifcall.mode & f.mode) != py9p.OREAD :
            raise py9p.ServerError("permission denied")
        srv.respond(req, None)
If we have a file tree then simply check whether the Qid matches
dataset/ETHPy150Open svinota/py9p/examples/composite.py/SampleFs.open