--- a/treepkg/status.py	Thu Mar 15 12:08:52 2007 +0100
+++ b/treepkg/status.py	Thu Mar 15 12:20:12 2007 +0100
@@ -53,9 +53,16 @@
 
 class Status(object):
 
-    magic = "Status 0.0\n"
+    # Overwrite in derived classes with a different magic string
+    _magic = "Status 0.0\n"
 
-    attrs = set(["_filename", "_values"])
+    # Derived classes may extend a copy of this set with more instance
+    # variables.
+    _attrs = set(["_filename", "_values"])
+
+    # Derived classes should set this to a suitable list of attribute
+    # descriptors
+    _fields = ()
 
     def __init__(self, filename):
         assert os.path.isabs(filename)
@@ -72,24 +79,24 @@
         f = open(self._filename)
         try:
             magic = f.next()
-            if magic != self.magic:
+            if magic != self._magic:
                 raise ValueError("File %r has wrong magic" % self._filename)
             for line in f:
                 field, value = line.split(":", 1)
-                self._values[field] = self.fields[field].deserialize(value)
+                self._values[field] = self._fields[field].deserialize(value)
         finally:
             f.close()
 
     def write(self):
-        lines = [self.magic]
-        for field, desc in self.fields.items():
+        lines = [self._magic]
+        for field, desc in self._fields.items():
             if field in self._values:
                 lines.append("%s: %s\n"
                              % (field, desc.serialize(self._values[field])))
         util.writefile(self._filename, "".join(lines))
 
     def __getattr__(self, attr):
-        desc = self.fields.get(attr)
+        desc = self._fields.get(attr)
         if desc is not None:
             if attr in self._values:
                 return self._values[attr]
@@ -98,10 +105,10 @@
         raise AttributeError(attr)
 
     def __setattr__(self, attr, value):
-        if attr in self.fields:
+        if attr in self._fields:
             self._values[attr] = value
             self.write()
-        elif attr in self.attrs:
+        elif attr in self._attrs:
             self.__dict__[attr] = value
         else:
             raise AttributeError(attr)
@@ -109,9 +116,9 @@
 
 class RevisionStatus(Status):
 
-    magic = "TreePackagerStatus 0.0\n"
+    _magic = "TreePackagerStatus 0.0\n"
 
-    fields = dict(status=StringFieldDesc(default="unknown"),
-                  start=DateFieldDesc(default=None),
-                  stop=DateFieldDesc(default=None))
+    _fields = dict(status=StringFieldDesc(default="unknown"),
+                   start=DateFieldDesc(default=None),
+                   stop=DateFieldDesc(default=None))