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Fix-up comments and add a sentinel variable for clarity.

This commit is contained in:
Raymond Hettinger 2012-03-30 21:50:40 -07:00
parent 41eb79a016
commit 7f7a5a7b87
1 changed files with 17 additions and 16 deletions
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19
Lib/functools.py
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@ -154,8 +154,8 @@ def lru_cache(maxsize=100, typed=False):
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().
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
@ -169,10 +169,11 @@ def lru_cache(maxsize=100, typed=False):
def decorating_function(user_function):
cache = dict()
cache = {}
hits = misses = 0
kwd_mark = (object(),) # separate positional and keyword args
cache_get = cache.get # bound method to lookup key or return None
sentinel = object() # unique object used with cache_get
_len = len # localize the global len() function
lock = Lock() # because linkedlist updates aren't threadsafe
root = [] # root of the circular doubly linked list
@ -180,7 +181,7 @@ def lru_cache(maxsize=100, typed=False):
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
# build a cache key from positional and keyword args
key = args
if kwds:
sorted_items = tuple(sorted(kwds.items()))
@ -194,7 +195,7 @@ def lru_cache(maxsize=100, typed=False):
if maxsize == 0:
def wrapper(*args, **kwds):
# no caching, just do a statistics update after a successful call
# no caching, just a statistics update after a successful call
nonlocal misses
result = user_function(*args, **kwds)
misses += 1
@ -206,8 +207,8 @@ def lru_cache(maxsize=100, typed=False):
# simple caching without ordering or size limit
nonlocal hits, misses
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:
result = cache_get(key, sentinel)
if result is not sentinel:
hits += 1
return result
result = user_function(*args, **kwds)
@ -224,7 +225,7 @@ def lru_cache(maxsize=100, typed=False):
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
# move the link to the front of the circular queue
link_prev, link_next, key, result = link
link_prev[NEXT] = link_next
link_next[PREV] = link_prev
@ -237,7 +238,7 @@ def lru_cache(maxsize=100, typed=False):
result = user_function(*args, **kwds)
with lock:
if _len(cache) < maxsize:
# put result in a new link at the front of the list
# put result in a new link at the front of the queue
last = root[PREV]
link = [last, root, key, result]
cache[key] = last[NEXT] = root[PREV] = link