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from collections import namedtuple
from functools import wraps
from threading import Lock

_CacheInfo = namedtuple("CacheInfo", ["hits", "misses", "maxsize", "currsize"])

def lru_cache(maxsize=100, typed=False):
    """Least-recently-used cache decorator.

    If *maxsize* is set to None, the LRU features are disabled and the cache
    can grow without bound.

    If *typed* is True, arguments of different types will be cached separately.
    For example, f(3.0) and f(3) will be treated as distinct calls with
    distinct results.

    Arguments to the cached function must be hashable.

    View the cache statistics named tuple (hits, misses, maxsize, currsize) with
    f.cache_info().  Clear the cache and statistics with f.cache_clear().
    Access the underlying function with f.__wrapped__.

    See:  http://en.wikipedia.org/wiki/Cache_algorithms#Least_Recently_Used

    # Users should only access the lru_cache through its public API:
    #       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).

    def decorating_function(user_function):

        cache = dict()
        stats = [0, 0]
        HITS, MISSES = 0, 1             # names for the stats fields
        kwd_mark = (object(),)          # separate positional and keyword args
        cache_get = cache.get           # bound method to lookup key or return None
        _len = len                      # localize the global len() function
        lock = Lock()                   # because linkedlist updates aren't threadsafe
        root = []                       # root of the circular doubly linked list
        root[:] = [root, root, None, None]      # initialize by pointing to self
        PREV, NEXT, KEY, RESULT = 0, 1, 2, 3    # names for the link fields

        def make_key(args, kwds, typed, tuple=tuple, sorted=sorted, type=type):
            # helper function to build a cache key from positional and keyword args
            key = args
            if kwds:
                sorted_items = tuple(sorted(kwds.items()))
                key += kwd_mark + sorted_items
            if typed:
                key += tuple(type(v) for v in args)
                if kwds:
                    key += tuple(type(v) for k, v in sorted_items)
            return key

        if maxsize == 0:

            def wrapper(*args, **kwds):
                # no caching, just do a statistics update after a successful call
                result = user_function(*args, **kwds)
                stats[MISSES] += 1
                return result

        elif maxsize is None:

            def wrapper(*args, **kwds):
                # simple caching without ordering or size limit
                key = make_key(args, kwds, typed) if kwds or typed else args
                result = cache_get(key, root)   # root used here as a unique not-found sentinel
                if result is not root:
                    stats[HITS] += 1
                    return result
                result = user_function(*args, **kwds)
                cache[key] = result
                stats[MISSES] += 1
                return result


            def wrapper(*args, **kwds):
                # size limited caching that tracks accesses by recency
                key = make_key(args, kwds, typed) if kwds or typed else args
                with lock:
                    link = cache_get(key)
                    if link is not None:
                        # record recent use of the key by moving it to the front of the list
                        link_prev, link_next, key, result = link
                        link_prev[NEXT] = link_next
                        link_next[PREV] = link_prev
                        last = root[PREV]
                        last[NEXT] = root[PREV] = link
                        link[PREV] = last
                        link[NEXT] = root
                        stats[HITS] += 1
                        return result
                result = user_function(*args, **kwds)
                with lock:
                    last = root[PREV]
                    link = [last, root, key, result]
                    cache[key] = last[NEXT] = root[PREV] = link
                    if _len(cache) > maxsize:
                        # purge least recently used cache entry
                        old_prev, old_next, old_key, old_result = root[NEXT]
                        root[NEXT] = old_next
                        old_next[PREV] = root
                        del cache[old_key]
                    stats[MISSES] += 1
                return result

        def cache_info():
            """Report cache statistics"""
            with lock:
                return _CacheInfo(stats[HITS], stats[MISSES], maxsize, len(cache))

        def cache_clear():
            """Clear the cache and cache statistics"""
            with lock:
                root[:] = [root, root, None, None]
                stats[:] = [0, 0]

        wrapper.__wrapped__ = user_function
        wrapper.cache_info = cache_info
        wrapper.cache_clear = cache_clear
        return wrapper

    return decorating_function

Diff to Previous Revision

--- revision 3 2012-03-17 23:41:29
+++ revision 4 2012-03-17 23:42:52
@@ -62,7 +62,7 @@
                 stats[MISSES] += 1
                 return result
-        if maxsize is None:
+        elif maxsize is None:
             def wrapper(*args, **kwds):