What is the right way to have custom instance attributes in Django models?

Question

I want to add custom attributes to instances of a Django model. These attributes should not be stored in the database. In any other class, the attributes would simply be initialized by the __init__ method.

I can already see three different ways to do it, and none of them are completely satisfying. I wonder if there is any better/more pythonic/djangoist way to do it?

1. Override the __init__ method: the syntax is a bit convoluted, but it works.

   from django.db.models import Model
   
   class Foo(Model):
     def __init__(self, *args, **kwargs):
        super(Model, self).__init__(*args, **kwargs)
        self.bar = 1
   

2. Use a Django post_init signal: this takes class code outside of the class definition, which is not very readable.

   from django.dispatch import receiver
   from django.db.models.signals import post_init
   @receiver(post_init, sender=Foo)
       def user_init(sender, instance, *args, **kwargs):
         instance.bar = 1
   

3. Use an instance method instead of an attribute: having a general exception raised as the default behaviour is a bit disturbing.

   class Foo(Model):
     def bar(self):
       try:
         return self.bar
       except:
         self.bar = 1
       return self.bar
   

Of these three choices, the first looks like the least worst to me. What do you think? Any alternative?

Answer 1

I would use the property decorator available in python

class Foo(Model):
    @property
    def bar(self):
        if not hasattr(self, '_bar'):
            self._bar = 1

        return self._bar

Then you can access that just like a property instead of invoking a function with ()

You could even get a little more straight forward with this by having the fallback built in, instead of stored

class Foo(Model):
    @property
    def bar(self):
        return getattr(self, '_bar', 1)

Answer 2

Overriding __init__ is the right way.