Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.xs4all.nl!newsfeed4.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: odd behavoiur seen
Date: Mon, 22 Jul 2013 21:47:33 +0200
Organization: None
References: <2013072219364160391-chrishinsley@gmailcom> <2013072219424735368-chrishinsley@gmailcom>
Mime-Version: 1.0
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7Bit
User-Agent: KNode/4.7.3
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.4987.1374522447.3114.python-list@python.org>
Lines: 44
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:51057

Chris Hinsley wrote:

> On 2013-07-22 18:36:41 +0000, Chris Hinsley said:
> 
>> Folks, I have this decorator:
>> 
>> def memoize(maxsize):
>>     def _memoize(func):
>>         lru_cache = {}
>>         lru_list = []
> 
> Other clues, I use it on a recursive function:
> 
> @memoize(64)
> def next_move(board, colour, alpha, beta, ply):
>     if ply <= 0:
>         return evaluate(board) * colour
>     for new_board in all_moves(board[:], colour):
>         score = -next_move(new_board, -colour, -beta, -alpha, ply - 1)
>         if score >= beta:
>             return score
>         if score > alpha:
>             alpha = score
>     return alpha
> 
> And I notice I don't get the strange problem on a non-recursive
> function ! Or at least I don't seam to.

That's indeed the problem: 

>             if len(lru_list) >= maxsize:
>                 del(lru_cache[lru_list[0]])
>                 del(lru_list[0])
>             ret = func(*args, **kwargs)
>             lru_cache[key] = ret
>             lru_list.append(key)

You delete a cached item, then call the original function which causes calls 
of the decorated function. This causes a length check which sees the already 
reduced length and decides that the cache is not yet full.

If you remove the oldest item after calling the original function you should 
be OK.