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.
If your related model is called Foo you can just do:
except Foo.DoesNotExist:
Django is amazing when its 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:
ObjectDoesNotExist and DoesNotExist
exception DoesNotExist
The DoesNotExist exception is raised when an object is not found for the given parameters of a query. Django provides a DoesNotExist exception as an attribute of each model class to identify the class of object that could not be found and to allow you to catch a particular model class with
try
/except
.
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.