diff --git a/mercurial/state.py b/mercurial/state.py
--- a/mercurial/state.py
+++ b/mercurial/state.py
@@ -46,42 +46,35 @@
         """
         self._repo = repo
         self.fname = fname
-        if not opts:
-            self.opts = {}
-        else:
-            self.opts = opts
-
-    def __nonzero__(self):
-        return self.exists()
-
-    def __getitem__(self, key):
-        return self.opts[key]
 
-    def __setitem__(self, key, value):
-        updates = {key: value}
-        self.opts.update(updates)
+    def read(self):
+        """read the existing state file and return a dict of data stored"""
+        return self._read()
 
-    def load(self):
-        """load the existing state file into the class object"""
-        op = self._read()
-        self.opts.update(op)
-
-    def addopts(self, opts):
-        """add more key-value pairs to the data stored by the object"""
-        self.opts.update(opts)
-
-    def save(self):
+    def save(self, version, data):
         """write all the state data stored to .hg/<filename> file
 
         we use third-party library cbor to serialize data to write in the file.
         """
+        try:
+            iv = int(version)
+        except ValueError:
+            raise error.ProgrammingError("version of state file should be"
+                                         " an integer")
+
         with self._repo.vfs(self.fname, 'wb', atomictemp=True) as fp:
+            fp.write('%d\n' % iv)
             cbor.dump(self.opts, fp)
 
     def _read(self):
         """reads the state file and returns a dictionary which contain
         data in the same format as it was before storing"""
         with self._repo.vfs(self.fname, 'rb') as fp:
+            try:
+                version = int(fp.readline())
+            except ValueError:
+                raise error.ProgrammingError("unknown version of state file"
+                                             " found")
             return cbor.load(fp)
 
     def delete(self):