calling hasattr(model_instance, fieldname) raises DoesNotExist when False

[monkut]

Using python 3.3 and django 1.6.

I've got a model (created from an AbstractBaseClass) and creating an instance of the model from some JSON, via serializers.deserialize(), then checking if the resulting parsed JSON model has the fields I'm expecting.

(thinking about it now, this seems like the wrong thing to do, since it may be expected that the resulting Model instance has the field, but takes the default value? but anyway...)

When I check to see if the parsed instance.object has the given attribute, via hasattr(), it's raising DoesNotExist, instead of returning the expected False.

Again, maybe it's ok that it doesn't return False... but I don't think hasattr() should raise an exception.

from django.contrib.auth.models import User

class BaseTask(models.Model):
    assigned_to = models.ForiegnKey(User)

    class Meta:
        abstract = True

class Task(BaseTask):
    pass

Python 3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:06:53) [MSC v.1600 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from myproject.myapp.models import Task
>>> from django.contrib.auth.models import User
>>> t = Task()
>>> hasattr(t, "assigned_to")
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "C:\Python33\lib\site-packages\django\db\models\fields\related.py", line 314, in __get__
    "%s has no %s." % (self.field.model.__name__, self.field.name))
django.contrib.auth.models.DoesNotExist: Task has no assigned_to.
>>> u = User.objects.get(pk=1)
>>>
>>> t.assigned_to = u
>>> hasattr(t, "assigned_to")
True
>>>

[Aymeric Augustin]

This is a bit tricky. ​Python's documentation for hasattr notes that it's "implemented by calling getattr(object, name) and seeing whether it raises an AttributeError or not". However, when no related object exists, Django raises DoesNotExist, not AttributeError.

Could we raise a RelatedObjectDoesNotExist exception inheriting both from DoesNotExist (for backwards compatibility) and AttributeError (to change the behavior according to your suggestion)?

I'm not entirely sure it's a good idea, but it's worth considering.

[Simon Charette]

Replying to aaugustin:

Could we raise a RelatedObjectDoesNotExist exception inheriting both from DoesNotExist (for backwards compatibility) and AttributeError (to change the behavior according to your suggestion)?

​Gave it a try and it seems worth considering. The full test suite passes on Python2.7 SQLite3.

[Aymeric Augustin]

The patch looks good to me.

It adds tests for both forwards and backwards relations.

I don't see a reason to make this new exception a public API. It's just making hasattr behave as expected. Therefore, no documentation is needed at this time.

It could be preferrable to define RelatedObjectDoesNotExist similarly for the forwards and backwards relations. Is there a downside to using a cached property for both? I value consistency a lot in this area of the code base, for reasons that must be obvious to you by now ;-) Once you've considered this question, ship it!

[Simon Charette]

Replying to aaugustin:

It could be preferrable to define RelatedObjectDoesNotExist similarly for the forwards and backwards relations. Is there a downside to using a cached property for both? I value consistency a lot in this area of the code base, for reasons that must be obvious to you by now ;-) Once you've considered this question, ship it!

No objection to use a cached_property in both cases.

I kept the one created at initialization time since I felt like the cached_property approach was kind of hackish, hence the ReverseSingleRelatedObjectDescriptor ​comment.

I'll add a comment to explain SingleRelatedObjectDescriptor.RelatedObjectDoesNotExist is also a cached_property for the sake of consistency.

[Simon Charette]

Opened a ​PR with the changes, I'll ship it if no objections are raised in the next days.

[Simon Charette <charette.s@…>]

In 75924cfa6dca95aa1f02e38802df285271dc7c14:

Fixed #21563 -- Single related object descriptors should work with hasattr.

Thanks to Aymeric Augustin for the review and Trac alias monkut for the report.