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？