South's frozen models are big liars

March 07, 2013 at 02:53 PM | Python, Django | View Comments

Consider South's orm object, http://south.readthedocs.org/en/latest/ormfreezing.html, which provides "frozen-in-time" versions of ORM models:

(Pdb++) from my_app.models import MyModel
(Pdb++) MyModel
<class 'my_app.models.MyModel'>
(Pdb++) orm['my_app.MyModel']
<class 'my_app.models.MyModel'>
(Pdb++) orm['my_app.MyModel'] == MyModel
False

Notice that the magic South frozen model appears to be in the same namespace as my real model: my_app.models!

This is at best stupid, and at worst potentially dangerous.

It makes perfect sense that South needs some mechanism for providing "frozen models" (ie, which match the database schema at the time of migration, of the current schema), but they are emphatically not the same as the real models (don't have any of the real methods, etc), so it should be made very clear that they are different! There are many ways this could be done, but one very simple step would be putting them in a different namespace, which makes it clear that these models are frozen, and references the migrations they belong to:

(Pdb++) orm['my_app.MyModel']
<class 'south.frozen_models.my_app.models_0003.MyModel'>

Additionally, this is a bad code smell which could potentially lead to bugs: because the "frozen model class" is lying about where it's from, any code which relies on the module path will do surprising things. In fact, it's such a big problem that pickle explicitly checks for this case:

(Pdb++) import pickle
(Pdb++) pickle.dumps(orm['my_app.MyModel'])
*** PicklingError: Can't pickle <class 'my_app.models.MyModel'>:
    it's not the same object as my_app.models.MyModel