How do you catch this exception?

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:

To catch this exception in general, you can do

from django.core.exceptions import ObjectDoesNotExist

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