Apparently, skipUnless() doesn't prevent the execution of

setUp() under 3.1.

Lib/test/test_zlib.py

@@ -66,6 +66,10 @@ class ChecksumTestCase(unittest.TestCase):
 # Issue #10276 - check that inputs >=4GB are handled correctly.
 class ChecksumBigBufferTestCase(unittest.TestCase):
 
+    @unittest.skipUnless(mmap, "mmap() is not available.")
+    @unittest.skipUnless(sys.maxsize > _4G, "Can't run on a 32-bit system.")
+    @unittest.skipUnless(support.is_resource_enabled("largefile"),
+                         "May use lots of disk space.")
     def setUp(self):
         with open(support.TESTFN, "wb+") as f:
             f.seek(_4G)
@@ -77,10 +81,6 @@ class ChecksumBigBufferTestCase(unittest.TestCase):
         self.mapping.close()
         support.unlink(support.TESTFN)
 
-    @unittest.skipUnless(mmap, "mmap() is not available.")
-    @unittest.skipUnless(sys.maxsize > _4G, "Can't run on a 32-bit system.")
-    @unittest.skipUnless(support.is_resource_enabled("largefile"),
-                         "May use lots of disk space.")
     def test_big_buffer(self):
         self.assertEqual(zlib.crc32(self.mapping), 3058686908)
         self.assertEqual(zlib.adler32(self.mapping), 82837919)