Improve comments and variable names.
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@ -17,7 +17,7 @@ try:
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from _thread import RLock
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except:
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class RLock:
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'Dummy reentrant lock'
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'Dummy reentrant lock for builds without threads'
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def __enter__(self): pass
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def __exit__(self, exctype, excinst, exctb): pass
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@ -146,6 +146,12 @@ except ImportError:
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_CacheInfo = namedtuple("CacheInfo", ["hits", "misses", "maxsize", "currsize"])
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class _HashedSeq(list):
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""" This class guarantees that hash() will be called no more than once
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per element. This is important because the lru_cache() will hash
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the key multiple times on a cache miss.
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"""
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__slots__ = 'hashvalue'
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def __init__(self, tup, hash=hash):
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@ -159,7 +165,16 @@ def _make_key(args, kwds, typed,
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kwd_mark = (object(),),
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fasttypes = {int, str, frozenset, type(None)},
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sorted=sorted, tuple=tuple, type=type, len=len):
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'Make a cache key from optionally typed positional and keyword arguments'
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"""Make a cache key from optionally typed positional and keyword arguments
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The key is constructed in a way that is flat as possible rather than
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as a nested structure that would take more memory.
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If there is only a single argument and its data type is known to cache
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its hash value, then that argument is returned without a wrapper. This
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saves space and improves lookup speed.
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"""
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key = args
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if kwds:
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sorted_items = sorted(kwds.items())
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@ -217,7 +232,7 @@ def lru_cache(maxsize=128, typed=False):
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if maxsize == 0:
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def wrapper(*args, **kwds):
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# no caching, just 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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@ -226,7 +241,7 @@ def lru_cache(maxsize=128, typed=False):
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elif maxsize is None:
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def wrapper(*args, **kwds):
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# simple caching without ordering or size limit
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# Simple caching without ordering or size limit
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nonlocal hits, misses, currsize
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key = make_key(args, kwds, typed)
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result = cache_get(key, sentinel)
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@ -242,14 +257,14 @@ def lru_cache(maxsize=128, typed=False):
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else:
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def wrapper(*args, **kwds):
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# size limited caching that tracks accesses by recency
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# Size limited caching that tracks accesses by recency
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nonlocal root, hits, misses, currsize, full
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key = make_key(args, kwds, typed)
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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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# 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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# 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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last = root[PREV]
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@ -261,26 +276,34 @@ def lru_cache(maxsize=128, typed=False):
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result = user_function(*args, **kwds)
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with lock:
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if key in cache:
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# getting here means that this same key was added to the
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# cache while the lock was released. since the link
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# Getting here means that this same key was added to the
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# cache while the lock was released. Since the link
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# update is already done, we need only return the
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# computed result and update the count of misses.
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pass
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elif full:
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# use the old root to store the new key and result
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# Use the old root to store the new key and result.
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oldroot = root
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oldroot[KEY] = key
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oldroot[RESULT] = result
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# empty the oldest link and make it the new root
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# Empty the oldest link and make it the new root.
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# Keep a reference to the old key and old result to
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# prevent their ref counts from going to zero during the
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# update. That will prevent potentially arbitrary object
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# clean-up code (i.e. __del__) from running while we're
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# still adjusting the links.
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root = oldroot[NEXT]
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oldkey = root[KEY]
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oldvalue = root[RESULT]
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oldresult = root[RESULT]
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root[KEY] = root[RESULT] = None
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# now update the cache dictionary for the new links
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# Now update the cache dictionary.
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del cache[oldkey]
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# Save the potentially reentrant cache[key] assignment
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# for last, after the root and links have been put in
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# a consistent state.
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cache[key] = oldroot
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else:
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# put result in a new link at the front of the queue
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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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last[NEXT] = root[PREV] = cache[key] = link
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