How do you catch this exception?

Question

This code is in django/db/models/fields.py It creates/defines an exception?

class ReverseSingleRelatedObjectDescriptor(six.with_metaclass(RenameRelatedObjectDescriptorMethods)):
    # This class provides the functionality that makes the related-object
    # managers available as attributes on a model class, for fields that have
    # a single "remote" value, on the class that defines the related field.
    # In the example "choice.poll", the poll attribute is a
    # ReverseSingleRelatedObjectDescriptor instance.
    def __init__(self, field_with_rel):
        self.field = field_with_rel
        self.cache_name = self.field.get_cache_name()

    @cached_property
    def RelatedObjectDoesNotExist(self):
        # The exception can't be created at initialization time since the
        # related model might not be resolved yet; `rel.to` might still be
        # a string model reference.
        return type(
            str('RelatedObjectDoesNotExist'),
            (self.field.rel.to.DoesNotExist, AttributeError),
            {}
        )

This is in django/db/models/fields/related.py it raises the said exception above:

def __get__(self, instance, instance_type=None):
    if instance is None:
        return self
    try:
        rel_obj = getattr(instance, self.cache_name)
    except AttributeError:
        val = self.field.get_local_related_value(instance)
        if None in val:
            rel_obj = None
        else:
            params = dict(
                (rh_field.attname, getattr(instance, lh_field.attname))
                for lh_field, rh_field in self.field.related_fields)
            qs = self.get_queryset(instance=instance)
            extra_filter = self.field.get_extra_descriptor_filter(instance)
            if isinstance(extra_filter, dict):
                params.update(extra_filter)
                qs = qs.filter(**params)
            else:
                qs = qs.filter(extra_filter, **params)
            # Assuming the database enforces foreign keys, this won't fail.
            rel_obj = qs.get()
            if not self.field.rel.multiple:
                setattr(rel_obj, self.field.related.get_cache_name(), instance)
        setattr(instance, self.cache_name, rel_obj)
    if rel_obj is None and not self.field.null:
        raise self.RelatedObjectDoesNotExist(
            "%s has no %s." % (self.field.model.__name__, self.field.name)
        )
    else:
        return rel_obj

The problem is that this code:

    try:
        val = getattr(obj, attr_name)
    except related.ReverseSingleRelatedObjectDescriptor.RelatedObjectDoesNotExist:
        val = None  # Does not catch the thrown exception
    except Exception as foo:
        print type(foo)  # Catches here, not above

won't catch that exception

>>>print type(foo)
<class 'django.db.models.fields.related.RelatedObjectDoesNotExist'>
>>>isinstance(foo, related.FieldDoesNotExist)
False

and

except related.RelatedObjectDoesNotExist:

Raises an AttributeError: 'module' object has no attribute 'RelatedObjectDoesNotExist'

>>>isinstance(foo, related.ReverseSingleRelatedObjectDescriptor.RelatedObjectDoesNotExist)
Traceback (most recent call last):
  File "<string>", line 1, in <fragment>
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

which is probably why.

Solution 1

If your related model is called Foo you can just do:

except Foo.DoesNotExist:

Django is amazing when it's not terrifying. RelatedObjectDoesNotExist is a property that returns a type that is figured out dynamically at runtime. That type uses self.field.rel.to.DoesNotExist as a base class.

According to Django documentation:

DoesNotExist

exception Model.DoesNotExist

This exception is raised by the ORM when an expected object is not found. For example, QuerySet.get() will raise it when no object is found for the given lookups.

Django provides a DoesNotExist exception as an attribute of each model class to identify the class of object that could not be found, allowing you to catch exceptions for a particular model class.

The exception is a subclass of django.core.exceptions.ObjectDoesNotExist.

This is the magic that makes that happen. Once the model has been built up, self.field.rel.to.DoesNotExist is the does-not-exist exception for that model.

Solution 2

If you don't want to import the related model class, you can:

except MyModel.related_field.RelatedObjectDoesNotExist:

or

except my_model_instance._meta.model.related_field.RelatedObjectDoesNotExist:

where related_field is the field name.

Solution 3

Let's say we have the following models:

class MainModel(Model):
    pass

class RelatedModel(Model):
    main = OneToOneField(MainModel, null=True, related_name="related")

You can get a RelatedObjectDoesNotExist exception with MainModel().related.

You have three options for catching this exception, which you can find by looking at .__class__.__mro__ of the exception:

1. MainModel.related.RelatedObjectDoesNotExist
2. RelatedModel.DoesNotExist
3. django.core.exceptions.ObjectDoesNotExist

MainModel.related.RelatedObjectDoesNotExist

RelatedObjectDoesNotExist is what the question is looking for, but is specific to a nullable OneToOneField:

try:
    # Your code here
except MainModel.related.RelatedObjectDoesNotExist:
    # Handle exception

RelatedModel.DoesNotExist

Model.DoesNotExist is the parent class of RelatedObjectDoesNotExist. To catch it requires you to be able to import the model in question, but is a more generically useful code pattern.

try:
    # Your code here
except OtherModel.DoesNotExist:
    # Handle exception

django.core.exceptions.ObjectDoesNotExist

ObjectDoesNotExist is the parent class of Model.DoesNotExist. This will catch this exception for any model, which is helpful if you don't know what model will raise the exception:

from django.core.exceptions import ObjectDoesNotExist

try:
    # Your code here
except ObjectDoesNotExist:
    # Handle exception

Solution 4

The RelatedObjectDoesNotExist exception is created dynamically at runtime. Here is the relevant code snippet for the ForwardManyToOneDescriptor and ReverseOneToOneDescriptor descriptors:

@cached_property
def RelatedObjectDoesNotExist(self):
    # The exception can't be created at initialization time since the
    # related model might not be resolved yet; `self.field.model` might
    # still be a string model reference.
    return type(
        'RelatedObjectDoesNotExist',
        (self.field.remote_field.model.DoesNotExist, AttributeError),
        {}
    )

So the exception inherits from <model name>.DoesNotExist and AttributeError. In fact, the complete MRO for this exception type is:

[<class 'django.db.models.fields.related_descriptors.RelatedObjectDoesNotExist'>, 
<class '<model module path>.DoesNotExist'>,
<class 'django.core.exceptions.ObjectDoesNotExist'>,
<class 'AttributeError'>,
<class 'Exception'>,
<class 'BaseException'>,
<class 'object'>]

The basic takeaway is you can catch <model name>.DoesNotExist, ObjectDoesNotExist (import from django.core.exceptions) or AttributeError, whatever makes the most sense in your context.

Solution 5

Little bit late but helpful for others.

2 ways to handle this.

1st :

When we need to catch exception

>>> from django.core.exceptions import ObjectDoesNotExist
>>> try:
>>>     p2.restaurant
>>> except ObjectDoesNotExist:
>>>     print("There is no restaurant here.")
There is no restaurant here.

2nd: When don't want to handle exception

>>> hasattr(p2, 'restaurant')
False

View more solutions

Comments

[John Zwinck]

How have you imported related?

[catherine]

use AttributeError instead of related.ReverseSingleRelatedObjectDescriptor.RelatedObjectDo‌​esNotExist

[boatcoder]

Yes @JohnZwinck I have imported related.

[boatcoder]

Because the bug was in DjangoRestFramework, and the model is somewhat hard to come by at that point. I settled for catching ObjectDoesNotExist.

[Giovanni Di Milia]

this is actually pretty useful in case you need to avoid circular imports. Thanks

[Jordan Reiter]

You can also use AttributeError, which under certain circumstances may be a better option (this error almost always occurs when you are accessing the "attribute" of a record, so this way you don't have to keep track whether this attribute corresponds to a record or not.

[Eric Blum]

I found this didn't actually catch the error as expected. The <Model>.DoesNotExist did

[Zags]

@EricBlum <Model>.DoesNotExist is a descendant of ObjectDoesNotExist, so that shouldn't happen. Can you dig into why it is happening or give more details about your code?

[Neil]

Yeaaaa okay. But why can't it just return None? Especially with one-to-one fields. Or is there a good reason?

[Michael P]

This is all because of how the hasattr logic was changed in python 3. In order to work with hasattr it must be an exception that derives from the AttributeError class. Yet it must be also an exception that comes from the Model.DoesNotExist class.

[Osama Abuhamdan]

hasattr() is the right way to do it I think