Issue 10593: Adopt Nick's suggestion for an lru_cache with maxsize=None.

Doc/library/functools.rst

@@ -32,7 +32,7 @@ The :mod:`functools` module defines the following functions:
    A compare function is any callable that accept two arguments, compares them,
    and returns a negative number for less-than, zero for equality, or a positive
    number for greater-than.  A key function is a callable that accepts one
-   argument and returns another value that indicates the position in the desired
+   argument and returns another value indicating the position in the desired
    collation sequence.
 
    Example::
@@ -51,10 +51,14 @@ The :mod:`functools` module defines the following functions:
    Since a dictionary is used to cache results, the positional and keyword
    arguments to the function must be hashable.
 
+   If *maxsize* is set to None, the LRU feature is disabled and the cache
+   can grow without bound.
+
    To help measure the effectiveness of the cache and tune the *maxsize*
    parameter, the wrapped function is instrumented with a :func:`cache_info`
    function that returns a :term:`named tuple` showing *hits*, *misses*,
-   *maxsize* and *currsize*.
+   *maxsize* and *currsize*.  In a multi-threaded environment, the hits
+   and misses are approximate.
 
    The decorator also provides a :func:`cache_clear` function for clearing or
    invalidating the cache.
@@ -89,12 +93,25 @@ The :mod:`functools` module defines the following functions:
         >>> print(get_pep.cache_info())
         CacheInfo(hits=3, misses=8, maxsize=20, currsize=8)
 
-   .. versionadded:: 3.2
+   Example of efficiently computing
+   `Fibonacci numbers <http://en.wikipedia.org/wiki/Fibonacci_number>`_
+   using a cache to implement a
+   `dynamic programming <http://en.wikipedia.org/wiki/Dynamic_programming>`_
+   technique::
+
+        @lru_cache(maxsize=None)
+        def fib(n):
+            if n < 2:
+                return n
+            return fib(n-1) + fib(n-2)
 
-   .. seealso::
+        >>> print([fib(n) for n in range(16)])
+        [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610]
 
-      Recipe for a `plain cache without the LRU feature
-      <http://code.activestate.com/recipes/577479-simple-caching-decorator/>`_.
+        >>> print(fib.cache_info())
+        CacheInfo(hits=28, misses=16, maxsize=None, currsize=16)
+
+   .. versionadded:: 3.2
 
 .. decorator:: total_ordering
 

Lib/functools.py

@@ -119,6 +119,9 @@ _CacheInfo = namedtuple("CacheInfo", "hits misses maxsize currsize")
 def lru_cache(maxsize=100):
     """Least-recently-used cache decorator.
 
+    If *maxsize* is set to None, the LRU features are disabled and the cache
+    can grow without bound.
+
     Arguments to the cached function must be hashable.
 
     View the cache statistics named tuple (hits, misses, maxsize, currsize) with
@@ -136,13 +139,32 @@ def lru_cache(maxsize=100):
     def decorating_function(user_function,
                 tuple=tuple, sorted=sorted, len=len, KeyError=KeyError):
 
-        cache = OrderedDict()           # ordered least recent to most recent
-        cache_popitem = cache.popitem
-        cache_renew = cache.move_to_end
         hits = misses = 0
         kwd_mark = object()             # separates positional and keyword args
         lock = Lock()
 
+        if maxsize is None:
+            cache = dict()              # simple cache without ordering or size limit
+
+            @wraps(user_function)
+            def wrapper(*args, **kwds):
+                nonlocal hits, misses
+                key = args
+                if kwds:
+                    key += (kwd_mark,) + tuple(sorted(kwds.items()))
+                try:
+                    result = cache[key]
+                    hits += 1
+                except KeyError:
+                    result = user_function(*args, **kwds)
+                    cache[key] = result
+                    misses += 1
+                return result
+        else:
+            cache = OrderedDict()       # ordered least recent to most recent
+            cache_popitem = cache.popitem
+            cache_renew = cache.move_to_end
+
             @wraps(user_function)
             def wrapper(*args, **kwds):
                 nonlocal hits, misses

Lib/test/test_functools.py

@@ -586,6 +586,20 @@ class TestLRU(unittest.TestCase):
         self.assertEqual(misses, 4)
         self.assertEqual(currsize, 2)
 
+    def test_lru_with_maxsize_none(self):
+        @functools.lru_cache(maxsize=None)
+        def fib(n):
+            if n < 2:
+                return n
+            return fib(n-1) + fib(n-2)
+        self.assertEqual([fib(n) for n in range(16)],
+            [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610])
+        self.assertEqual(fib.cache_info(),
+            functools._CacheInfo(hits=28, misses=16, maxsize=None, currsize=16))
+        fib.cache_clear()
+        self.assertEqual(fib.cache_info(),
+            functools._CacheInfo(hits=0, misses=0, maxsize=None, currsize=0))
+
 def test_main(verbose=None):
     test_classes = (
         TestPartial,