new file mode 100644
--- /dev/null
+++ b/babel/messages/catalog.py
@@ -0,0 +1,121 @@
+# -*- coding: utf-8 -*-
+#
+# Copyright (C) 2007 Edgewall Software
+# All rights reserved.
+#
+# This software is licensed as described in the file COPYING, which
+# you should have received as part of this distribution. The terms
+# are also available at http://babel.edgewall.org/wiki/License.
+#
+# This software consists of voluntary contributions made by many
+# individuals. For the exact contribution history, see the revision
+# history and logs, available at http://babel.edgewall.org/log/.
+
+"""Data structures for message catalogs."""
+
+import re
+try:
+    set
+except NameError:
+    from sets import Set as set
+
+from babel.util import odict
+
+__all__ = ['Message', 'Catalog']
+__docformat__ = 'restructuredtext en'
+
+PYTHON_FORMAT = re.compile(r'\%(\([\w]+\))?[diouxXeEfFgGcrs]').search
+
+
+class Message(object):
+    """Representation of a single message in a catalog."""
+
+    def __init__(self, id, string=None, locations=(), flags=()):
+        """Create the message object.
+        
+        :param id: the message ID, or a ``(singular, plural)`` tuple for
+                   pluralizable messages
+        :param string: the translated message string, or a
+                       ``(singular, plural)`` tuple for pluralizable messages
+        :param locations: a sequence of ``(filenname, lineno)`` tuples
+        :param flags: a set or sequence of flags
+        """
+        self.id = id
+        self.string = string
+        self.locations = locations
+        self.flags = set(flags)
+        if self.python_format:
+            self.flags.add('python-format')
+        else:
+            self.flags.discard('python-format')
+
+    def __repr__(self):
+        return '<%s %r>' % (type(self).__name__, self.id)
+
+    def pluralizable(self):
+        return isinstance(self.id, (list, tuple))
+    pluralizable = property(pluralizable, doc="""\
+        Whether the message is plurizable.
+        
+        >>> Message('foo').pluralizable
+        False
+        >>> Message(('foo', 'bar')).pluralizable
+        True
+        
+        :rtype:  `bool`
+        """)
+
+    def python_format(self):
+        ids = self.id
+        if not isinstance(ids, (list, tuple)):
+            ids = [ids]
+        return bool(filter(None, [PYTHON_FORMAT(id) for id in ids]))
+    python_format = property(python_format, doc="""\
+        Whether the message contains Python-style parameters.
+        
+        >>> Message('foo %(name)s bar').python_format
+        True
+        >>> Message(('foo %(name)s', 'foo %(name)s')).python_format
+        True
+        
+        :rtype:  `bool`
+        """)
+
+
+class Catalog(object):
+    """Representation a message catalog."""
+
+    def __init__(self, domain=None):
+        self.domain = domain
+        self.messages = odict()
+
+    def __iter__(self):
+        for id in self.messages:
+            yield self.messages[id]
+
+    def __repr__(self):
+        return '<%s %r>' % (type(self).__name__, self.domain)
+
+    def __delitem__(self, id):
+        if id in self.messaages:
+            del self.messages[id]
+
+    def __getitem__(self, id):
+        return self.messages.get(id)
+
+    def __setitem__(self, id, message):
+        assert isinstance(message, Message), 'expected a Message object'
+        current = self.messages.get(id)
+        if current:
+            assert current.string == message.string, 'translation mismatch'
+            current.locations.extend(message.locations)
+            current.flags |= message.flags
+            message = current
+        else:
+            if isinstance(id, (list, tuple)):
+                singular, plural = id
+                id = singular
+            self.messages[id] = message
+
+    def add(self, id, string=None, locations=(), flags=()):
+        self[id] = Message(id, string, locations, flags)