--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/app/django/db/models/fields/subclassing.py	Fri Jul 18 18:22:23 2008 +0000
@@ -0,0 +1,53 @@
+"""
+Convenience routines for creating non-trivial Field subclasses.
+
+Add SubfieldBase as the __metaclass__ for your Field subclass, implement
+to_python() and the other necessary methods and everything will work seamlessly.
+"""
+
+from django.utils.maxlength import LegacyMaxlength
+
+class SubfieldBase(LegacyMaxlength):
+    """
+    A metaclass for custom Field subclasses. This ensures the model's attribute
+    has the descriptor protocol attached to it.
+    """
+    def __new__(cls, base, name, attrs):
+        new_class = super(SubfieldBase, cls).__new__(cls, base, name, attrs)
+        new_class.contribute_to_class = make_contrib(
+                attrs.get('contribute_to_class'))
+        return new_class
+
+class Creator(object):
+    """
+    A placeholder class that provides a way to set the attribute on the model.
+    """
+    def __init__(self, field):
+        self.field = field
+
+    def __get__(self, obj, type=None):
+        if obj is None:
+            raise AttributeError('Can only be accessed via an instance.')
+        return obj.__dict__[self.field.name]        
+
+    def __set__(self, obj, value):
+        obj.__dict__[self.field.name] = self.field.to_python(value)
+
+def make_contrib(func=None):
+    """
+    Returns a suitable contribute_to_class() method for the Field subclass.
+
+    If 'func' is passed in, it is the existing contribute_to_class() method on
+    the subclass and it is called before anything else. It is assumed in this
+    case that the existing contribute_to_class() calls all the necessary
+    superclass methods.
+    """
+    def contribute_to_class(self, cls, name):
+        if func:
+            func(self, cls, name)
+        else:
+            super(self.__class__, self).contribute_to_class(cls, name)
+        setattr(cls, self.name, Creator(self))
+
+    return contribute_to_class
+