fix testing multiple backends

The decorator for rerunning with a different backend did not take the
side effects into account.
Create different TestCases objects instead.

test.py

@@ -11,31 +11,35 @@
 import vignette
 
 
-def do_all_backends(func):
-	@wraps(func)
-	def decorator(*args):
-		assert vignette.get_metadata_backend(), 'there are no testable backends'
+ALL_THUMBNAIL = vignette.THUMBNAILER_BACKENDS
+IMAGE_THUMBNAIL = [vignette.QtBackend(), vignette.PilBackend(), vignette.MagickBackend()]
+IMAGE_THUMBNAIL = [b for b in IMAGE_THUMBNAIL if b.is_available()]
 
-		all_backends = vignette.METADATA_BACKENDS
-		for backend in all_backends:
-			if backend.is_available():
-				vignette.METADATA_BACKENDS = [backend]
-				try:
-					func(*args)
-				except:
-					logging.warning('backend %r failed', type(backend))
-					raise
-		vignette.METADATA_BACKENDS = all_backends
-
-	return decorator
+ALL_METADATA = vignette.METADATA_BACKENDS
+AVAIL_METADATA = [b for b in ALL_METADATA if b.is_available()]
 
 
 class ThumbnailTests(unittest.TestCase):
+	def __init__(self, metadata=None, thumbnail=None, *args, **kwargs):
+		super(ThumbnailTests, self).__init__(*args, **kwargs)
+		self.metadata_backends = metadata
+		self.thumbnail_backends = thumbnail
+
 	def setUp(self):
 		self.dir = os.environ['XDG_CACHE_HOME'] = tempfile.mkdtemp()
 		self.filename = os.path.join(os.environ['XDG_CACHE_HOME'], 'test.png')
 		shutil.copyfile('test.png', self.filename)
 
+		if self.metadata_backends is None:
+			vignette.METADATA_BACKENDS = ALL_METADATA
+		else:
+			vignette.METADATA_BACKENDS = self.metadata_backends
+
+		if self.thumbnail_backends is None:
+			vignette.THUMBNAILER_BACKENDS = ALL_THUMBNAIL
+		else:
+			vignette.THUMBNAILER_BACKENDS = self.thumbnail_backends
+
 	def tearDown(self):
 		shutil.rmtree(self.dir)
 
@@ -44,7 +48,6 @@ def test_hash(self):
 		dest = os.path.join(self.dir, 'thumbnails', 'large', hashlib.md5(uri.encode('utf-8')).hexdigest()) + '.png'
 		self.assertEqual(dest, vignette.build_thumbnail_path(self.filename, 'large'))
 
-	@do_all_backends
 	def test_basic(self):
 		dest = vignette.build_thumbnail_path(self.filename, 'large')
 		assert dest
@@ -139,6 +142,22 @@ def test_put_fail(self):
 		self.assertEqual(dest, vignette.get_thumbnail(self.filename, use_fail_appname='foo'))
 
 
+class MultiBackendsLoader(unittest.TestLoader):
+	def loadTestsFromTestCase(self, testCaseClass):
+		testCaseNames = self.getTestCaseNames(testCaseClass)
+		tests = []
+		for name in testCaseNames:
+			tests.append(testCaseClass(methodName=name))
+			for b in AVAIL_METADATA:
+				tests.append(testCaseClass(metadata=[b], methodName=name))
+
+			for b in IMAGE_THUMBNAIL:
+				tests.append(testCaseClass(thumbnail=[b], methodName=name))
+
+		loaded_suite = self.suiteClass(tests)
+		return loaded_suite
+
+
 if __name__ == '__main__':
 	logging.basicConfig()
-	unittest.main()
+	unittest.main(testLoader=MultiBackendsLoader())