Django model instance __nonzero__ override ignored

I'm trying to override a model instance's __nonzero__ so it evaluates to False

I've first tested on a simple Python class:

>>> class A():
...     pass
... 
>>> a = A()
>>> a.__nonzero__ = lambda: False
>>> b = A()
>>> bool(a)
False
>>> bool(b)
True
>>> type(a).__nonzero__(a)
False
>>> type(b).__nonzero__(b)
True

This shows that __nonzero__ being overridden successfully alters the object's bool() evaluation.

I then tried to apply the same thing to a django model object (instance of feincms.module.page.Page that itself inherits from `django.db.models.Manager).

from django.contrib import admin
from django.utils.translation import ugettext_lazy as _
from feincms.module.page.modeladmins import PageAdmin
from feincms.module.page.models import Page
from guardian.admin import GuardedModelAdmin
from reversion import VersionAdmin


class PageAdmin(PageAdmin, GuardedModelAdmin, VersionAdmin):

    # some stuff
    # ...
    #

    def render_revision_form(self, *args, **kwargs): 
        """Override render_revision_form so we can make ``obj`` evaluate to False
        so ``feincms.module.page.forms.PageAdminForm.clean()`` does not try
        to query ``self.page_manager.get(pk=current_id)`` where current_id
        is ``obj.pk`` in the below function.
        This is needed because ``self.page_manager.get(pk=current_id)`` is
        queried on feincms Page objects rather than reversion Version objects.
        """
        request, obj, version, context = args
        obj.__nonzero__ = lambda: False
        obj.hello = 'a'
        import pdb
        pdb.set_trace()
        return super(PageAdmin, self).render_revision_form(request, obj, version, context, **kwargs)

admin.site.register(Page, PageAdmin)

but this evaluates to True:

(Pdb) obj
<Page: test>
(Pdb) bool(obj)
True

Is there any reason that am I not aware of that could cause this behaviour?

You are using old-style class in this example. And model instance is a new-style class.

class B(object): pass
b = B()
b.__nonzero__ = lambda: False
bool(b) # True

Apparently, you can't override special methods for the instances, only for types.

B.__nonzero__ = lambda self: False
bool(b) # False

Read the following SO question for examples and more detailed explanation: "overriding-special-methods-on-an-instance"