diff --git a/tests/test_CacheWriter.py b/tests/test_CacheWriter.py
index 1ceadce..4023a4e 100644
--- a/tests/test_CacheWriter.py
+++ b/tests/test_CacheWriter.py
@@ -15,12 +15,9 @@ class TestCacheFile(oxrlib.cache.WriteCacheFile):
     def __init__(self, path, *args, **kwargs):
         self.path = path
         assert args[0].startswith('w')
-
-    def open(self):
-        last_file = StringIO()
-        last_file.name = self.path.as_posix()
-        type(self).last_file = last_file
-        return self.last_file
+        self.open_file = StringIO()
+        self.open_file.name = self.path.as_posix()
+        type(self).last_file = self.open_file
 
 
 class FakeRate: