diff -r e8447da8791d Lib/test/test_codecs.py
--- a/Lib/test/test_codecs.py	Sat Sep 06 20:38:23 2014 +1000
+++ b/Lib/test/test_codecs.py	Sat Sep 06 22:40:48 2014 +1000
@@ -2602,7 +2602,14 @@
         self.obj_to_raise = RuntimeError
 
     def tearDown(self):
-        _TEST_CODECS.pop(self.codec_name, None)
+        entry = _TEST_CODECS.pop(self.codec_name, None)
+        encodings._cache.pop(self.codec_name, None)
+        if entry is not None:
+            # Issue #22166: Also need to clear the internal cache in CPython
+            import gc
+            for ref in gc.get_referrers(entry):
+                if isinstance(ref, dict):
+                    ref.pop(self.codec_name, None)
 
     def set_codec(self, encode, decode):
         codec_info = codecs.CodecInfo(encode, decode,