16

我正在一个需要查询 PostgreSQL 数据库的 Django Web 应用程序中工作。使用 Python线程接口实现并发时DoesNotExist，查询项出现错误。当然，按顺序执行查询时不会发生这些错误。

让我展示一个我编写的单元测试来演示意外行为：

class ThreadingTest(TestCase):
    fixtures = ['demo_city',]

    def test_sequential_requests(self):
        """
        A very simple request to database, made sequentially.

        A fixture for the cities has been loaded above. It is supposed to be
        six cities in the testing database now. We will made a request for
        each one of the cities sequentially.
        """
        for number in range(1, 7):
            c = City.objects.get(pk=number)
            self.assertEqual(c.pk, number)

    def test_threaded_requests(self):
        """
        Now, to test the threaded behavior, we will spawn a thread for
        retrieving each city from the database.
        """

        threads = []
        cities = []

        def do_requests(number):
            cities.append(City.objects.get(pk=number))

        [threads.append(threading.Thread(target=do_requests, args=(n,))) for n in range(1, 7)]

        [t.start() for t in threads]
        [t.join() for t in threads]

        self.assertNotEqual(cities, [])

如您所见，第一个测试按顺序执行了一些数据库请求，这些请求确实没有问题。然而，第二个测试执行完全相同的请求，但每个请求都在一个线程中产生。这实际上是失败的，返回一个DoesNotExist异常。

这个单元测试的执行输出是这样的：

test_sequential_requests (cesta.core.tests.threadbase.ThreadingTest) ... ok
test_threaded_requests (cesta.core.tests.threadbase.ThreadingTest) ...

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.6/threading.py", line 484, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/jose/Work/cesta/trunk/src/cesta/core/tests/threadbase.py", line 45, in do_requests
    cities.append(City.objects.get(pk=number))
  File "/home/jose/Work/cesta/trunk/parts/django/django/db/models/manager.py", line 132, in get
    return self.get_query_set().get(*args, **kwargs)
  File "/home/jose/Work/cesta/trunk/parts/django/django/db/models/query.py", line 349, in get
    % self.model._meta.object_name)
DoesNotExist: City matching query does not exist.

...其他线程返回类似的输出...

Exception in thread Thread-6:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.6/threading.py", line 484, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/jose/Work/cesta/trunk/src/cesta/core/tests/threadbase.py", line 45, in do_requests
    cities.append(City.objects.get(pk=number))
  File "/home/jose/Work/cesta/trunk/parts/django/django/db/models/manager.py", line 132, in get
    return self.get_query_set().get(*args, **kwargs)
  File "/home/jose/Work/cesta/trunk/parts/django/django/db/models/query.py", line 349, in get
    % self.model._meta.object_name)
DoesNotExist: City matching query does not exist.


FAIL

======================================================================
FAIL: test_threaded_requests (cesta.core.tests.threadbase.ThreadingTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jose/Work/cesta/trunk/src/cesta/core/tests/threadbase.py", line 52, in test_threaded_requests
    self.assertNotEqual(cities, [])
AssertionError: [] == []

----------------------------------------------------------------------
Ran 2 tests in 0.278s

FAILED (failures=1)
Destroying test database for alias 'default' ('test_cesta')...

请记住，所有这些都发生在 PostgreSQL 数据库中，它应该是线程安全的，而不是 SQLite 或类似的。测试也是使用 PostgreSQL 运行的。

在这一点上，我完全不知道什么会失败。有什么想法或建议吗？

谢谢！

编辑：我写了一个小视图只是为了检查它是否在测试中有效。这是视图的代码：

def get_cities(request):
    queue = Queue.Queue()

    def get_async_cities(q, n):
        city = City.objects.get(pk=n)
        q.put(city)

    threads = [threading.Thread(target=get_async_cities, args=(queue, number)) for number in range(1, 5)]

    [t.start() for t in threads]
    [t.join() for t in threads]

    cities = list()

    while not queue.empty():
        cities.append(queue.get())

    return render_to_response('async/cities.html', {'cities': cities},
        context_instance=RequestContext(request))

（请不要考虑在视图代码中编写应用程序逻辑的愚蠢行为。请记住，这只是概念验证，不会出现在实际应用程序中。）

结果是代码运行良好，请求在线程中成功发出，视图在调用其 URL 后最终显示城市。

所以，我认为只有在需要测试代码时才使用线程进行查询。在生产中，它将毫无问题地工作。

有什么有用的建议可以成功测试这种代码吗？

4

3 回答 3

16

尝试使用 TransactionTestCase：

class ThreadingTest(TransactionTestCase):

TestCase 将数据保存在内存中，并且不会向数据库发出 COMMIT。可能线程正在尝试直接连接到数据库，而数据尚未在那里提交。此处的说明： https ://docs.djangoproject.com/en/dev/topics/testing/?from=olddocs#django.test.TransactionTestCase

TransactionTestCase 和 TestCase 是相同的，除了将数据库重置为已知状态的方式以及测试代码测试提交和回滚效果的能力。TransactionTestCase 在测试运行之前通过截断所有表并重新加载初始数据来重置数据库。TransactionTestCase 可以调用 commit 和 rollback 并观察这些调用对数据库的影响。

于 2012-06-08T13:16:35.033 回答
3

从文档的这一部分变得更加清晰

class LiveServerTestCase(TransactionTestCase):
    """
    ...
    Note that it inherits from TransactionTestCase instead of TestCase because
    the threads do not share the same transactions (unless if using in-memory
    sqlite) and each thread needs to commit all their transactions so that the
    other thread can see the changes.
    """

现在，事务尚未在 TestCase 中提交，因此其他线程看不到更改。

于 2014-02-22T07:13:43.230 回答
2

这听起来像是交易的问题。如果您在当前请求（或测试）中创建元素，则几乎可以肯定它们处于未提交的事务中，无法从另一个线程中的单独连接访问。您可能需要手动管理您的交易才能使其正常工作。

于 2012-06-08T12:09:30.473 回答