Issue 13177: Make tracebacks more readable by avoiding chained exceptions in the lru_cache.

Lib/functools.py

@@ -141,7 +141,7 @@ def lru_cache(maxsize=100):
 
         hits = misses = 0
         kwd_mark = (object(),)          # separates positional and keyword args
-        lock = Lock()                   # needed because ordereddicts aren't threadsafe
+        lock = Lock()                   # needed because OrderedDict isn't threadsafe
 
         if maxsize is None:
             cache = dict()              # simple cache without ordering or size limit
@@ -155,7 +155,9 @@ def lru_cache(maxsize=100):
                 try:
                     result = cache[key]
                     hits += 1
+                    return result
                 except KeyError:
+                    pass
                 result = user_function(*args, **kwds)
                 cache[key] = result
                 misses += 1
@@ -171,12 +173,14 @@ def lru_cache(maxsize=100):
                 key = args
                 if kwds:
                     key += kwd_mark + tuple(sorted(kwds.items()))
-                try:
                 with lock:
+                    try:
                         result = cache[key]
                         cache_renew(key)    # record recent use of this key
                         hits += 1
+                        return result
                     except KeyError:
+                        pass
                 result = user_function(*args, **kwds)
                 with lock:
                     cache[key] = result     # record recent use of this key

Lib/test/test_functools.py

@@ -655,6 +655,22 @@ class TestLRU(unittest.TestCase):
         self.assertEqual(fib.cache_info(),
             functools._CacheInfo(hits=0, misses=0, maxsize=None, currsize=0))
 
+    def test_lru_with_exceptions(self):
+        # Verify that user_function exceptions get passed through without
+        # creating a hard-to-read chained exception.
+        # http://bugs.python.org/issue13177
+        for maxsize in (None, 100):
+            @functools.lru_cache(maxsize)
+            def func(i):
+                return 'abc'[i]
+            self.assertEqual(func(0), 'a')
+            with self.assertRaises(IndexError) as cm:
+                func(15)
+            self.assertIsNone(cm.exception.__context__)
+            # Verify that the previous exception did not result in a cached entry
+            with self.assertRaises(IndexError):
+                func(15)
+
 def test_main(verbose=None):
     test_classes = (
         TestPartial,

Misc/NEWS

@@ -46,6 +46,10 @@ Library
 - Issue #13158: Fix decoding and encoding of GNU tar specific base-256 number
   fields in tarfile.
 
+- Issue #13177:  Functools lru_cache() no longer calls the original function
+  inside an exception handler.  This makes tracebacks easier to read because
+  chained exceptions are avoided.
+
 - Issue #13025: mimetypes is now reading MIME types using the UTF-8 encoding,
   instead of the locale encoding.