How do I get one class method to be different for each child (and call it from base class)

Question

With an instance of Concert I get: unbound method do_stuff() must be called with Concert instance as first argument (got ModelBase instance instead)

models.py:

class Event(models.Model):
    def do_stuff(self):
        response self.do_specific_stuff(self)


class Concert(Event):
    def do_specific_stuff(self):
        ...

class Party(Event):
    def do_specific_stuff(self):
        ...

views:

def index(request):
    x = Concert.objects.get(name='Jack White @ Oakland')
    output = x.do_stuff()
    return HttpResponse(output)

My goal is to loop trough all the events and execute the do_specific_stuff child class method based on what kind of event it is.

Your class Event has no method do_specific_stuff(), and if it had, you dont need to pass self to it. — PepperoniPizza, Nov 20 '12 at 15:03
Which class does `Event.objects.get(name='Jack White @ Oakland')` return an instance of? — Vaughn Cato, Nov 20 '12 at 15:09
I know but my question is can I call the subclass method do_specific_stuff (of the concert in this example) trough the base class in views.py? — Dieter, Nov 20 '12 at 15:10
@Dieter: The Event class doesn't have do_specific_stuff. You will need to get either a Concert or Party instance. — Vaughn Cato, Nov 20 '12 at 15:13

score 3 · Accepted Answer · edited May 23 '17 at 11:56

In Django, inheritance triggers multi-table inheritance, but you don't get the polymorphism in Python. It's just an instance of the ORM not providing a perfect correspondence between the data schema and the object model.

In other words, when you query Event, you get back a whole bunch of Event objects, regardless of whether some of them are actually Concert or Party objects. You have to manually downcast. If an Event is a Concert, it will have an attribute called concert, which points to the corresponding Concert subclass. Likewise for Party. If it is just a normal Event, it will have neither attribute.

You could use the following property on Event to automatically downcast your object:

@property
def as_child_class(self):
    """Casts this object to its subclass, if possible"""

    if hasattr(self, 'concert'):
        return self.concert
    elif hasattr(self, 'party'):
        return self.party
    else:
        return self

Then you could do something like:

for event in Event.objects.all()
    event.as_child_class.do_specific_stuff()

Similar questions have come up before:

And this link has some other ideas:

http://jeffelmore.org/2010/11/11/automatic-downcasting-of-inherited-models-in-django/

Only if you're calling it on Event objects would you want to do `self.as_child_class.do_specific_stuff()`. You don't need to manually pass self to any of the functions, and because `as_child_class` is a property, you can treat it like you're retrieving a member variable. Check my edit out, for code you might call from a view. — acjay, Nov 20 '12 at 15:38

Gerard · Answer 2 · 2012-11-20T15:54:46.750

It seems to me that your Event model is ONLY for inheritance, so you should abstract it:

class Event(models.Model):
    class Meta:
        absract = True

    def do_stuff(self):
        response self.do_specific_stuff()

    def do_specific_stuff(self):
        raise NotImplemented


class Concert(Event):
    def do_specific_stuff(self):
        ...


class Party(Event):
    def do_specific_stuff(self):
        ...

I may be wrong about your usage of Event, but if it was, abstracting your Event model will have it to be like a plain class, I mean no database actions will be taken for such model.

Hope this helps! :)

score 0 · Answer 3 · answered Nov 20 '12 at 16:18

First of all, see Template method

Secondly, the Event class should be abstract.

class Event:
   def __init__:
       raise NotImplemented('This class is abstract')

Thirdly, see Single Table Inheritance and Class Table Inheritance patterns. And django-ORM realisation of the patterns there Good luck =)

How do I get one class method to be different for each child (and call it from base class)

3 Answers3