Improve comments and variable names.

This commit is contained in:
Raymond Hettinger 2013-03-08 21:11:55 -07:00
parent b6f1d6419d
commit f96b2b03bb
1 changed files with 37 additions and 14 deletions

View File

@ -17,7 +17,7 @@ try:
from _thread import RLock
except:
class RLock:
'Dummy reentrant lock'
'Dummy reentrant lock for builds without threads'
def __enter__(self): pass
def __exit__(self, exctype, excinst, exctb): pass
@ -146,6 +146,12 @@ except ImportError:
_CacheInfo = namedtuple("CacheInfo", ["hits", "misses", "maxsize", "currsize"])
class _HashedSeq(list):
""" This class guarantees that hash() will be called no more than once
per element. This is important because the lru_cache() will hash
the key multiple times on a cache miss.
"""
__slots__ = 'hashvalue'
def __init__(self, tup, hash=hash):
@ -159,7 +165,16 @@ def _make_key(args, kwds, typed,
kwd_mark = (object(),),
fasttypes = {int, str, frozenset, type(None)},
sorted=sorted, tuple=tuple, type=type, len=len):
'Make a cache key from optionally typed positional and keyword arguments'
"""Make a cache key from optionally typed positional and keyword arguments
The key is constructed in a way that is flat as possible rather than
as a nested structure that would take more memory.
If there is only a single argument and its data type is known to cache
its hash value, then that argument is returned without a wrapper. This
saves space and improves lookup speed.
"""
key = args
if kwds:
sorted_items = sorted(kwds.items())
@ -217,7 +232,7 @@ def lru_cache(maxsize=128, typed=False):
if maxsize == 0:
def wrapper(*args, **kwds):
# no caching, just 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
@ -226,7 +241,7 @@ def lru_cache(maxsize=128, typed=False):
elif maxsize is None:
def wrapper(*args, **kwds):
# simple caching without ordering or size limit
# Simple caching without ordering or size limit
nonlocal hits, misses, currsize
key = make_key(args, kwds, typed)
result = cache_get(key, sentinel)
@ -242,14 +257,14 @@ def lru_cache(maxsize=128, typed=False):
else:
def wrapper(*args, **kwds):
# size limited caching that tracks accesses by recency
# Size limited caching that tracks accesses by recency
nonlocal root, hits, misses, currsize, full
key = make_key(args, kwds, typed)
with lock:
link = cache_get(key)
if link is not None:
# move the link to the front of the circular queue
link_prev, link_next, key, result = link
# 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
last = root[PREV]
@ -261,26 +276,34 @@ def lru_cache(maxsize=128, typed=False):
result = user_function(*args, **kwds)
with lock:
if key in cache:
# getting here means that this same key was added to the
# cache while the lock was released. since the link
# Getting here means that this same key was added to the
# cache while the lock was released. Since the link
# update is already done, we need only return the
# computed result and update the count of misses.
pass
elif full:
# use the old root to store the new key and result
# Use the old root to store the new key and result.
oldroot = root
oldroot[KEY] = key
oldroot[RESULT] = result
# empty the oldest link and make it the new root
# Empty the oldest link and make it the new root.
# Keep a reference to the old key and old result to
# prevent their ref counts from going to zero during the
# update. That will prevent potentially arbitrary object
# clean-up code (i.e. __del__) from running while we're
# still adjusting the links.
root = oldroot[NEXT]
oldkey = root[KEY]
oldvalue = root[RESULT]
oldresult = root[RESULT]
root[KEY] = root[RESULT] = None
# now update the cache dictionary for the new links
# Now update the cache dictionary.
del cache[oldkey]
# Save the potentially reentrant cache[key] assignment
# for last, after the root and links have been put in
# a consistent state.
cache[key] = oldroot
else:
# put result in a new link at the front of the queue
# Put result in a new link at the front of the queue.
last = root[PREV]
link = [last, root, key, result]
last[NEXT] = root[PREV] = cache[key] = link