Python 你如何捕捉这个异常？

How do you catch this exception?

提问by boatcoder

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

这段代码在 django/db/models/fields.py 它创建/定义了一个异常？

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:

这是在 django/db/models/fields/related.py 它引发了上述异常：

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:

问题是这段代码：

    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

不会捕捉到那个异常

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

and

和

except related.RelatedObjectDoesNotExist:

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

提出一个 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.

这可能是为什么。

采纳答案by tdelaney

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

如果你的相关模型被称为 Foo 你可以这样做：

except Foo.DoesNotExist:

Django is amazing when its not terrifying. RelatedObjectDoesNotExistis a property that returns a type that is figured out dynamically at runtime. That type uses self.field.rel.to.DoesNotExistas a base class. According to Django documentation:

Django 在它不可怕的时候是惊人的。RelatedObjectDoesNotExist是返回在运行时动态计算出的类型的属性。该类型self.field.rel.to.DoesNotExist用作基类。根据 Django 文档：

ObjectDoesNotExist and DoesNotExist

exception DoesNotExist

The DoesNotExistexception is raised when an object is not found for the given parameters of a query. Django provides a DoesNotExistexception 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.

ObjectDoesNotExist 和DoesNotExist

异常不存在

该DoesNotExist时未找到查询的给定参数的对象异常。Django 提供了一个DoesNotExist异常作为每个模型类的属性，以标识无法找到的对象的类，并允许您使用try/except捕获特定的模型类。

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

这就是让这一切发生的魔法。一旦模型建立起来，self.field.rel.to.DoesNotExist就是那个模型不存在的例外。

回答by Fush

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

如果不想导入相关的模型类，可以：

except MyModel.related_field.RelatedObjectDoesNotExist:

or

或者

except my_model_instance._meta.model.related_field.RelatedObjectDoesNotExist:

where related_fieldis the field name.

related_field字段名在哪里。

回答by Zags

To catch this exception in general, you can do

要在一般情况下捕获此异常，您可以执行

from django.core.exceptions import ObjectDoesNotExist

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

回答by LisaD

tdelaney's answer is great for regular code paths, but if you need to know how to catch this exception in tests:

tdelaney 的答案非常适合常规代码路径，但如果您需要知道如何在测试中捕获此异常：

from django.core.exceptions import ObjectDoesNotExist

...

    def testCompanyRequired(self):
        with self.assertRaises(ObjectDoesNotExist):
            employee = Employee.objects.create()

回答by C S

The RelatedObjectDoesNotExistexception is created dynamically at runtime. Here is the relevant code snippet for the ForwardManyToOneDescriptorand ReverseOneToOneDescriptordescriptors:

将RelatedObjectDoesNotExist在运行时动态创建的例外。以下是ForwardManyToOneDescriptor和ReverseOneToOneDescriptor描述符的相关代码片段：

@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>.DoesNotExistand AttributeError. In fact, the complete MRO for this exception type is:

所以异常继承自<model name>.DoesNotExistand AttributeError。事实上，这种异常类型的完整 MRO 是：

[<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.

基本要点是您可以 catch <model name>.DoesNotExist, ObjectDoesNotExist(import from django.core.exceptions) or AttributeError，任何在您的上下文中最有意义的东西。

回答by Muhammad Faizan Fareed

Little bit late but helpful for others.

有点晚，但对其他人有帮助。

2 ways to handle this.

2种方法来处理这个。

1st :

第一：

When we need to catch exception

当我们需要捕获异常时

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

>>> 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

第二：当不想处理异常时

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

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