diff --git a/scrapy/pqueues.py b/scrapy/pqueues.py
index 0681e6729..6ecd1b51a 100644
--- a/scrapy/pqueues.py
+++ b/scrapy/pqueues.py
@@ -11,7 +11,16 @@ logger = logging.getLogger(__name__)
 
 
 def _path_safe(text):
-    """ Return a filesystem-safe version of a string ``text`` """
+    """
+    Return a filesystem-safe version of a string ``text``
+
+    >>> _path_safe('simple.org').startswith('simple.org')
+    True
+    >>> _path_safe('dash-underscore_.org').startswith('dash-underscore_.org')
+    True
+    >>> _path_safe('some@symbol?').startswith('some_symbol_')
+    True
+    """
     pathable_slot = "".join([c if c.isalnum() or c in '-._' else '_'
                              for c in text])
     # as we replace some letters we can get collision for different slots
@@ -131,7 +140,6 @@ class DownloaderAwarePriorityQueue(object):
     domains (slots) with the least amount of active downloads are dequeued
     first.
     """
-    _DOWNLOADER_AWARE_PQ_ID = '_DOWNLOADER_AWARE_PQ_ID'
 
     @classmethod
     def from_crawler(cls, crawler, qfactory, slot_startprios=None, serialize=False):
diff --git a/tests/test_scheduler.py b/tests/test_scheduler.py
index eaf748d35..e0e3600e5 100644
--- a/tests/test_scheduler.py
+++ b/tests/test_scheduler.py
@@ -20,7 +20,7 @@ MockEngine = collections.namedtuple('MockEngine', ['downloader'])
 MockSlot = collections.namedtuple('MockSlot', ['active'])
 
 
-class MockDownloader:
+class MockDownloader(object):
     def __init__(self):
         self.slots = dict()
 
@@ -57,7 +57,7 @@ class MockCrawler(Crawler):
         self.engine = MockEngine(downloader=MockDownloader())
 
 
-class SchedulerHandler:
+class SchedulerHandler(object):
     priority_queue_cls = None
     jobdir = None
 
@@ -220,34 +220,6 @@ class TestMigration(unittest.TestCase):
             self._migration(self.tmpdir)
 
 
-class TestSchedulerWithDownloaderAwareInMemory(BaseSchedulerInMemoryTester,
-                                               unittest.TestCase):
-    priority_queue_cls = 'scrapy.pqueues.DownloaderAwarePriorityQueue'
-
-    def test_logic(self):
-        for url, slot in _URLS_WITH_SLOTS:
-            request = Request(url)
-            request.meta[Downloader.DOWNLOAD_SLOT] = slot
-            self.scheduler.enqueue_request(request)
-
-        downloader = self.mock_crawler.engine.downloader
-        dequeued_slots = list()
-        requests = list()
-        while self.scheduler.has_pending_requests():
-            request = self.scheduler.next_request()
-            slot = downloader._get_slot_key(request, None)
-            dequeued_slots.append(slot)
-            downloader.increment(slot)
-            requests.append(request)
-
-        for request in requests:
-            slot = downloader._get_slot_key(request, None)
-            self.mock_crawler.engine.downloader.decrement(slot)
-
-        self.assertTrue(_is_scheduling_fair(list(s for u, s in _URLS_WITH_SLOTS),
-                                            dequeued_slots))
-
-
 def _is_scheduling_fair(enqueued_slots, dequeued_slots):
     """
     We enqueued same number of requests for every slot.
@@ -273,31 +245,33 @@ def _is_scheduling_fair(enqueued_slots, dequeued_slots):
     return True
 
 
-class TestSchedulerWithDownloaderAwareOnDisk(BaseSchedulerOnDiskTester,
-                                             unittest.TestCase):
+class DownloaderAwareSchedulerTestMixin(object):
     priority_queue_cls = 'scrapy.pqueues.DownloaderAwarePriorityQueue'
+    reopen = False
 
     def test_logic(self):
-
         for url, slot in _URLS_WITH_SLOTS:
             request = Request(url)
             request.meta[Downloader.DOWNLOAD_SLOT] = slot
             self.scheduler.enqueue_request(request)
 
-        self.close_scheduler()
-        self.create_scheduler()
+        if self.reopen:
+            self.close_scheduler()
+            self.create_scheduler()
 
         dequeued_slots = list()
         requests = []
         downloader = self.mock_crawler.engine.downloader
         while self.scheduler.has_pending_requests():
             request = self.scheduler.next_request()
+            # pylint: disable=protected-access
             slot = downloader._get_slot_key(request, None)
             dequeued_slots.append(slot)
             downloader.increment(slot)
             requests.append(request)
 
         for request in requests:
+            # pylint: disable=protected-access
             slot = downloader._get_slot_key(request, None)
             downloader.decrement(slot)
 
@@ -306,10 +280,23 @@ class TestSchedulerWithDownloaderAwareOnDisk(BaseSchedulerOnDiskTester,
         self.assertEqual(sum(len(s.active) for s in downloader.slots.values()), 0)
 
 
+class TestSchedulerWithDownloaderAwareInMemory(DownloaderAwareSchedulerTestMixin,
+                                               BaseSchedulerInMemoryTester,
+                                               unittest.TestCase):
+    pass
+
+
+class TestSchedulerWithDownloaderAwareOnDisk(DownloaderAwareSchedulerTestMixin,
+                                             BaseSchedulerOnDiskTester,
+                                             unittest.TestCase):
+    reopen = True
+
+
 class StartUrlsSpider(Spider):
 
     def __init__(self, start_urls):
         self.start_urls = start_urls
+        super(StartUrlsSpider, self).__init__(start_urls)
 
     def parse(self, response):
         pass