Issue 10586: change the new functools.lru_cache implementation to expose the maximum and current cache sizes through the public statistics API. This API is now a single function that returns a named tuple.

Doc/library/functools.rst

@@ -52,10 +52,16 @@ The :mod:`functools` module defines the following functions:
    results, the positional and keyword arguments to the function must be
    hashable.
 
-   The wrapped function is instrumented with two attributes, :attr:`cache_hits`
-   and :attr:`cache_misses` which count the number of successful or unsuccessful
-   cache lookups.  These statistics are helpful for tuning the *maxsize*
-   parameter and for measuring the cache's effectiveness.
+   The wrapped function is instrumented with a :attr:`cache_info` attribute that
+   can be called to retrieve a named tuple with the following fields:
+
+      - :attr:`maxsize`: maximum cache size (as set by the *maxsize* parameter)
+      - :attr:`size`: current number of entries in the cache
+      - :attr:`hits`: number of successful cache lookups
+      - :attr:`misses`: number of unsuccessful cache lookups.
+
+   These statistics are helpful for tuning the *maxsize* parameter and for measuring
+   the effectiveness of the cache.
 
    The wrapped function also has a :attr:`cache_clear` attribute which can be
    called (with no arguments) to clear the cache.

Doc/whatsnew/3.2.rst

@@ -332,6 +332,8 @@ New, Improved, and Deprecated Modules
          c.execute('SELECT phonenumber FROM phonelist WHERE name=?', (name,))
          return c.fetchone()[0]
 
+  XXX: update for Issue 10586 changes to cache statistics API
+
   To help with choosing an effective cache size, the wrapped function is
   instrumented with two attributes *cache_hits* and *cache_misses*:
 

Lib/functools.py

@@ -12,7 +12,7 @@ __all__ = ['update_wrapper', 'wraps', 'WRAPPER_ASSIGNMENTS', 'WRAPPER_UPDATES',
            'total_ordering', 'cmp_to_key', 'lru_cache', 'reduce', 'partial']
 
 from _functools import partial, reduce
-from collections import OrderedDict
+from collections import OrderedDict, namedtuple
 try:
     from _thread import allocate_lock as Lock
 except:
@@ -114,12 +114,15 @@ def cmp_to_key(mycmp):
             raise TypeError('hash not implemented')
     return K
 
+_CacheInfo = namedtuple("CacheInfo", "maxsize, size, hits, misses")
+
 def lru_cache(maxsize=100):
     """Least-recently-used cache decorator.
 
     Arguments to the cached function must be hashable.
 
-    Performance statistics stored in f.cache_hits and f.cache_misses.
+    Significant statistics (maxsize, size, hits, misses) are
+      available through the f.cache_info() named tuple.
     Clear the cache and statistics using f.cache_clear().
     The underlying function is stored in f.__wrapped__.
 
@@ -127,7 +130,7 @@ def lru_cache(maxsize=100):
 
     """
     # Users should only access the lru_cache through its public API:
-    #   cache_hits, cache_misses, cache_clear(), and __wrapped__
+    #   cache_info, cache_clear, and f.__wrapped__
     # The internals of the lru_cache are encapsulated for thread safety and
     # to allow the implementation to change (including a possible C version).
 
@@ -137,11 +140,13 @@ def lru_cache(maxsize=100):
         cache = OrderedDict()           # ordered least recent to most recent
         cache_popitem = cache.popitem
         cache_renew = cache.move_to_end
+        hits = misses = 0
         kwd_mark = object()             # separate positional and keyword args
         lock = Lock()
 
         @wraps(user_function)
         def wrapper(*args, **kwds):
+            nonlocal hits, misses
             key = args
             if kwds:
                 key += (kwd_mark,) + tuple(sorted(kwds.items()))
@@ -149,23 +154,29 @@ def lru_cache(maxsize=100):
                 with lock:
                     result = cache[key]
                     cache_renew(key)            # record recent use of this key
-                    wrapper.cache_hits += 1
+                    hits += 1
             except KeyError:
                 result = user_function(*args, **kwds)
                 with lock:
                     cache[key] = result         # record recent use of this key
-                    wrapper.cache_misses += 1
+                    misses += 1
                     if len(cache) > maxsize:
                         cache_popitem(0)        # purge least recently used cache entry
             return result
 
+        def cache_info():
+            """Report significant cache statistics"""
+            with lock:
+                return _CacheInfo(maxsize, len(cache), hits, misses)
+
         def cache_clear():
             """Clear the cache and cache statistics"""
+            nonlocal hits, misses
             with lock:
                 cache.clear()
-                wrapper.cache_hits = wrapper.cache_misses = 0
+                hits = misses = 0
 
-        wrapper.cache_hits = wrapper.cache_misses = 0
+        wrapper.cache_info = cache_info
         wrapper.cache_clear = cache_clear
         return wrapper
 

Lib/test/test_functools.py

@@ -501,6 +501,11 @@ class TestLRU(unittest.TestCase):
         def orig(x, y):
             return 3*x+y
         f = functools.lru_cache(maxsize=20)(orig)
+        maxsize, currsize, hits, misses = f.cache_info()
+        self.assertEqual(maxsize, 20)
+        self.assertEqual(currsize, 0)
+        self.assertEqual(hits, 0)
+        self.assertEqual(misses, 0)
 
         domain = range(5)
         for i in range(1000):
@@ -508,21 +513,29 @@ class TestLRU(unittest.TestCase):
             actual = f(x, y)
             expected = orig(x, y)
             self.assertEqual(actual, expected)
-        self.assertTrue(f.cache_hits > f.cache_misses)
-        self.assertEqual(f.cache_hits + f.cache_misses, 1000)
+        maxsize, currsize, hits, misses = f.cache_info()
+        self.assertTrue(hits > misses)
+        self.assertEqual(hits + misses, 1000)
+        self.assertEqual(currsize, 20)
 
         f.cache_clear()   # test clearing
-        self.assertEqual(f.cache_hits, 0)
-        self.assertEqual(f.cache_misses, 0)
+        maxsize, currsize, hits, misses = f.cache_info()
+        self.assertEqual(hits, 0)
+        self.assertEqual(misses, 0)
+        self.assertEqual(currsize, 0)
         f(x, y)
-        self.assertEqual(f.cache_hits, 0)
-        self.assertEqual(f.cache_misses, 1)
+        maxsize, currsize, hits, misses = f.cache_info()
+        self.assertEqual(hits, 0)
+        self.assertEqual(misses, 1)
+        self.assertEqual(currsize, 1)
 
         # Test bypassing the cache
         self.assertIs(f.__wrapped__, orig)
         f.__wrapped__(x, y)
-        self.assertEqual(f.cache_hits, 0)
-        self.assertEqual(f.cache_misses, 1)
+        maxsize, currsize, hits, misses = f.cache_info()
+        self.assertEqual(hits, 0)
+        self.assertEqual(misses, 1)
+        self.assertEqual(currsize, 1)
 
         # test size zero (which means "never-cache")
         @functools.lru_cache(0)
@@ -530,10 +543,15 @@ class TestLRU(unittest.TestCase):
             nonlocal f_cnt
             f_cnt += 1
             return 20
+        self.assertEqual(f.cache_info().maxsize, 0)
         f_cnt = 0
         for i in range(5):
             self.assertEqual(f(), 20)
         self.assertEqual(f_cnt, 5)
+        maxsize, currsize, hits, misses = f.cache_info()
+        self.assertEqual(hits, 0)
+        self.assertEqual(misses, 5)
+        self.assertEqual(currsize, 0)
 
         # test size one
         @functools.lru_cache(1)
@@ -541,10 +559,15 @@ class TestLRU(unittest.TestCase):
             nonlocal f_cnt
             f_cnt += 1
             return 20
+        self.assertEqual(f.cache_info().maxsize, 1)
         f_cnt = 0
         for i in range(5):
             self.assertEqual(f(), 20)
         self.assertEqual(f_cnt, 1)
+        maxsize, currsize, hits, misses = f.cache_info()
+        self.assertEqual(hits, 4)
+        self.assertEqual(misses, 1)
+        self.assertEqual(currsize, 1)
 
         # test size two
         @functools.lru_cache(2)
@@ -552,11 +575,16 @@ class TestLRU(unittest.TestCase):
             nonlocal f_cnt
             f_cnt += 1
             return x*10
+        self.assertEqual(f.cache_info().maxsize, 2)
         f_cnt = 0
         for x in 7, 9, 7, 9, 7, 9, 8, 8, 8, 9, 9, 9, 8, 8, 8, 7:
             #    *  *              *                          *
             self.assertEqual(f(x), x*10)
         self.assertEqual(f_cnt, 4)
+        maxsize, currsize, hits, misses = f.cache_info()
+        self.assertEqual(hits, 12)
+        self.assertEqual(misses, 4)
+        self.assertEqual(currsize, 2)
 
 def test_main(verbose=None):
     test_classes = (

Misc/NEWS

@@ -43,6 +43,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #10586: The statistics API for the new functools.lru_cache has
+  been changed to a single cache_info() method returning a named tuple.
+
 - Issue #10323: itertools.islice() now consumes the minimum number of
   inputs before stopping.  Formerly, the final state of the underlying
   iterator was undefined.