Improve error reporting for downloads

test/test_download.py

@@ -125,11 +125,14 @@ def generator(test_case):
                     self.assertEqual(md5_for_file, tc['md5'])
                 with io.open(tc['file'] + '.info.json', encoding='utf-8') as infof:
                     info_dict = json.load(infof)
-                for (info_field, value) in tc.get('info_dict', {}).items():
+                for (info_field, expected) in tc.get('info_dict', {}).items():
-                    if isinstance(value, compat_str) and value.startswith('md5:'):
+                    if isinstance(expected, compat_str) and expected.startswith('md5:'):
-                        self.assertEqual(value, 'md5:' + md5(info_dict.get(info_field)))
+                        self.assertEqual(expected, 'md5:' + md5(info_dict.get(info_field)))
                     else:
-                        self.assertEqual(value, info_dict.get(info_field), u'invalid value for field ' + info_field)
+                        got = info_dict.get(info_field)
+                        self.assertEqual(
+                            expected, got,
+                            u'invalid value for field %s, expected %r, got %r' % (info_field, expected, got))
 
                 # If checkable fields are missing from the test case, print the info_dict
                 test_info_dict = dict((key, value if not isinstance(value, compat_str) or len(value) < 250 else 'md5:' + md5(value))