lrucache.py

from collections import namedtuple
from functools import update_wrapper
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]                  # make statistics updateable non-locally
        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
        nonlocal_root = [root]                  # make updateable non-locally
        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

        else:

            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
                        root, = nonlocal_root
                        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:
                    root = nonlocal_root[0]
                    if _len(cache) < maxsize:
                        # put result in a new link at the front of the list
                        last = root[PREV]
                        link = [last, root, key, result]
                        cache[key] = last[NEXT] = root[PREV] = link
                    else:
                        # use root to store the new key and result
                        root[KEY] = key
                        root[RESULT] = result
                        cache[key] = root
                        # empty the oldest link and make it the new root
                        root = nonlocal_root[0] = root[NEXT]
                        del cache[root[KEY]]
                        root[KEY] = None
                        root[RESULT] = None
                    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:
                cache.clear()
                root = nonlocal_root[0]
                root[:] = [root, root, None, None]
                stats[:] = [0, 0]

        wrapper.__wrapped__ = user_function
        wrapper.cache_info = cache_info
        wrapper.cache_clear = cache_clear
        return update_wrapper(wrapper, user_function)

    return decorating_function

@lru_cache(maxsize=None)
def fib(n):
    if n < 2:
        return n
    return fib(n-1) + fib(n-2)

def main():
    print([fib(n) for n in range(16)])
    print(fib.cache_info())

if __name__ == "__main__":
    main()