Django: Where does "DoesNotExist" come from?

Question

All the time in Django I see DoesNotExist being raised like in db.models.fields.related.py. Not ObjectDoesNotExist which is defined in django.core.exceptions, but just DoesNotExist. Where is this exception class defined, or am I not fully understanding exceptions? I've checked that it's not in exceptions (at least not that I know of). I'm confused obviously.

Note: It also comes free, as an attribute of a model sub-class instance, like `self.someforeignkey.DoesNotExist. How is this possible?

Answer 1

DoesNotExist, just like MultipleObjectsReturned, is an attribute on a model class. It is added to the model when the model is generated.

EDIT: The metaclass responsible for this process is ModelBase (django.db.models.base) and you may look at it to see what is going on.

Answer 2

DoesNotExist is documented here:

The DoesNotExist exception inherits from django.core.exceptions.ObjectDoesNotExist, so you can target multiple DoesNotExist exceptions.

so you can perfectly well use except ObjectDoesNotExist: and catch all the model-specific DoesNotExist exceptions that might be raised in the try clause, or use except SomeSpecificModel.DoesNotExist: when you want to be more specific.

If you're looking for the specific spot in Django's source code where this attribute is added to model classes, see here, lines 34-37:

# Create the class.
new_class = type.__new__(cls, name, bases, {'__module__': attrs.pop('__module__')})
new_class.add_to_class('_meta', Options(attrs.pop('Meta', None)))
new_class.add_to_class('DoesNotExist', types.ClassType('DoesNotExist', (ObjectDoesNotExist,), {}))