Fixed {Dict,List}Field default issue. Closes #46.

mongoengine/fields.py

@@ -263,7 +263,7 @@ class ListField(BaseField):
             raise ValidationError('Argument to ListField constructor must be '
                                   'a valid field')
         self.field = field
-        kwargs.setdefault('default', [])
+        kwargs.setdefault('default', lambda: [])
         super(ListField, self).__init__(**kwargs)
 
     def __get__(self, instance, owner):
@@ -356,7 +356,7 @@ class DictField(BaseField):
     def __init__(self, basecls=None, *args, **kwargs):
         self.basecls = basecls or BaseField
         assert issubclass(self.basecls, BaseField)
-        kwargs.setdefault('default', {})
+        kwargs.setdefault('default', lambda: {})
         super(DictField, self).__init__(*args, **kwargs)
 
     def validate(self, value):

tests/fields.py

@@ -693,5 +693,18 @@ class FieldTest(unittest.TestCase):
 
         Event.drop_collection()
 
+    def test_ensure_unique_default_instances(self):
+        """Ensure that every document has it's own unique default instance."""
+        class D(Document):
+            data = DictField()
+            data2 = DictField(default=lambda: {})
+
+        d1 = D()
+        d1.data['foo'] = 'bar'
+        d1.data2['foo'] = 'bar'
+        d2 = D()
+        self.assertEqual(d2.data, {})
+        self.assertEqual(d2.data2, {})
+
 if __name__ == '__main__':
     unittest.main()