remove previous_status from StatusField public API

I may be missing the use case, but this seems too ephemeral to be
useful (only exists on same instance after a save, can't rely on it
being there in general). If it's just an implementation detail for
StatusModifiedField, it doesn't need to be tested in public API for
StatusField.

model_utils/fields.py

@@ -42,6 +42,7 @@ def _previous_status(model_instance, attname, add):
         return None
     return getattr(current, attname, None)
 
+
 class StatusField(models.CharField):
     """
     A CharField that has set status choices by default.
@@ -54,17 +55,12 @@ class StatusField(models.CharField):
     def contribute_to_class(self, cls, name):
         if not cls._meta.abstract:
             assert hasattr(cls, 'STATUS'), \
-                "The model '%s' doesn't have status choices." % cls.__name__
+                "To use StatusField, the model '%s' must have a STATUS choices attribute." \
+                % cls.__name__
             setattr(self, '_choices', cls.STATUS)
             setattr(self, 'default', tuple(cls.STATUS)[0][0]) # sets first as default
         super(StatusField, self).contribute_to_class(cls, name)
 
-    def pre_save(self, model_instance, add):
-        previous = _previous_status(model_instance, 'get_%s_display' % self.attname, add)
-        if previous:
-            previous = previous()
-        setattr(model_instance, 'previous_status', previous)
-        return super(StatusField, self).pre_save(model_instance, add)
 
 class StatusModifiedField(models.DateTimeField):
 

model_utils/tests/tests.py

@@ -179,13 +179,7 @@ class StatusModelTests(TestCase):
         t1.save()
         self.assert_(t1.status_date > date_active_again)
 
-    def testPreviousConditon(self):
-        status = self.model.objects.create()
-        self.assertEquals(status.previous_status, None)
-        status.status = status.STATUS.on_hold
-        status.save()
-        self.assertEquals(status.previous_status, status.STATUS.active)
-
+        
 class Status2ModelTests(StatusModelTests):
     def setUp(self):
         self.model = Status2
@@ -206,12 +200,6 @@ class Status2ModelTests(StatusModelTests):
         t1.save()
         self.assert_(t1.status_date > date_active_again)
 
-    def testPreviousConditon(self):
-        status = self.model.objects.create()
-        self.assertEquals(status.previous_status, None)
-        status.status = status.STATUS[2][0]
-        status.save()
-        self.assertEquals(status.previous_status, status.STATUS[0][0])
 
 class QueryManagerTests(TestCase):
     def setUp(self):