Admin documentation on views suggests poor way of adding extra_context

[Chris Pratt]

As reference see: ​https://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.change_view

If you pass my_context for extra_context in this way, it completely drops any existing context passed in from a subclass also overriding this method. This could lead to a confusing scenario in which Django is seemingly ignoring extra_context. One better approach would be:

def change_view(self, request, object_id, extra_context=None):
    extra_context = extra_context or {}
    extra_context['osm_data'] = self.get_osm_info()
    return super(MyModelAdmin, self).change_view(request, object_id,
            extra_context=extra_context)

It's a relatively small difference, I'll admit, but since the docs are probably most frequently read by those new to Django and possibly even Python itself, I believe the sample code should take into account eventualities such as the ModelAdmin being subclassed.

[Claude Paroz]

Documentation fix

[Tim Graham]

In [17582]:

Fixed #17749 - Documented better way of overriding ModelAdmin; thanks chrisdpratt and claudep.

[Tim Graham]

In [17583]:

[1.3.X] Fixed #17749 - Documented better way of overriding ModelAdmin; thanks chrisdpratt and claudep.

Backport of r17582 from trunk.