问为什么Django单元测试没有看到抛出的异常？

EN

# models.py
class List(models.Model):
    owner = models.ForeignKey(User)
    type = models.ForeignKey('ListType', help_text=_('Type of list'))
    name = models.CharField(_('list'), max_length=128, help_text=_('Name of list'))

class ListType(models.Model):
    type = models.CharField(_('type'), max_length=16)

@staticmethod
def read_list(list_id, list_name, owner, list_type):
    try:
        return List.objects.get(pk=list_id, name=list_name, owner=owner, type=list_type)
    except List.DoesNotExist:
        return None

# tests.py
from django.test import TestCase
from .factories import *
from .models import List, ListType

class TestFuncs(TestCase):
    def test_read_list_exc(self):
        with self.assertRaises(List.DoesNotExist):
            uf = UserFactory()
            lt = ListType.objects.get(type='Member')
            lf = ListFactory(owner=uf, type=lt, name='foo')
            # I've created one list but its name isn't 'bar'
            list = List.read_list(999, 'bar', uf, lt)

# set_trace output:
(<class 'list.models.DoesNotExist'>, DoesNotExist('List matching query does not exist.',))

# test output:
...
File "...."
    list = List.read_list(999, 'bar', uf, lt)
AssertionError: DoesNotExist not raised

        ...
        with self.assertRaises(list.models.DoesNotExist):
        ...