Fix-up comments and add a sentinel variable for clarity.
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@ -154,8 +154,8 @@ def lru_cache(maxsize=100, typed=False):
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Arguments to the cached function must be hashable.
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View the cache statistics named tuple (hits, misses, maxsize, currsize) with
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f.cache_info(). Clear the cache and statistics with f.cache_clear().
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View the cache statistics named tuple (hits, misses, maxsize, currsize)
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with f.cache_info(). Clear the cache and statistics with f.cache_clear().
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Access the underlying function with f.__wrapped__.
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See: http://en.wikipedia.org/wiki/Cache_algorithms#Least_Recently_Used
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@ -169,18 +169,19 @@ def lru_cache(maxsize=100, typed=False):
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def decorating_function(user_function):
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cache = dict()
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cache = {}
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hits = misses = 0
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kwd_mark = (object(),) # separate positional and keyword args
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cache_get = cache.get # bound method to lookup key or return None
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_len = len # localize the global len() function
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lock = Lock() # because linkedlist updates aren't threadsafe
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root = [] # root of the circular doubly linked list
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root[:] = [root, root, None, None] # initialize by pointing to self
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PREV, NEXT, KEY, RESULT = 0, 1, 2, 3 # names for the link fields
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kwd_mark = (object(),) # separate positional and keyword args
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cache_get = cache.get # bound method to lookup key or return None
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sentinel = object() # unique object used with cache_get
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_len = len # localize the global len() function
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lock = Lock() # because linkedlist updates aren't threadsafe
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root = [] # root of the circular doubly linked list
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root[:] = [root, root, None, None] # initialize by pointing to self
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PREV, NEXT, KEY, RESULT = 0, 1, 2, 3 # names for the link fields
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def make_key(args, kwds, typed, tuple=tuple, sorted=sorted, type=type):
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# helper function to build a cache key from positional and keyword args
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# build a cache key from positional and keyword args
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key = args
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if kwds:
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sorted_items = tuple(sorted(kwds.items()))
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@ -194,7 +195,7 @@ def lru_cache(maxsize=100, typed=False):
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if maxsize == 0:
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def wrapper(*args, **kwds):
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# no caching, just do a statistics update after a successful call
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# no caching, just a statistics update after a successful call
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nonlocal misses
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result = user_function(*args, **kwds)
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misses += 1
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@ -206,8 +207,8 @@ def lru_cache(maxsize=100, typed=False):
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# simple caching without ordering or size limit
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nonlocal hits, misses
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key = make_key(args, kwds, typed) if kwds or typed else args
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result = cache_get(key, root) # root used here as a unique not-found sentinel
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if result is not root:
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result = cache_get(key, sentinel)
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if result is not sentinel:
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hits += 1
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return result
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result = user_function(*args, **kwds)
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@ -224,7 +225,7 @@ def lru_cache(maxsize=100, typed=False):
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with lock:
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link = cache_get(key)
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if link is not None:
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# record recent use of the key by moving it to the front of the list
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# move the link to the front of the circular queue
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link_prev, link_next, key, result = link
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link_prev[NEXT] = link_next
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link_next[PREV] = link_prev
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@ -237,7 +238,7 @@ def lru_cache(maxsize=100, typed=False):
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result = user_function(*args, **kwds)
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with lock:
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if _len(cache) < maxsize:
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# put result in a new link at the front of the list
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# put result in a new link at the front of the queue
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last = root[PREV]
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link = [last, root, key, result]
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cache[key] = last[NEXT] = root[PREV] = link
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