LRU cache

[Rob Cliffe]

On 18/02/2023 17:19, Albert-Jan Roskam wrote:
>
>
> On Feb 18, 2023 17:28, Rob Cliffe via Python-list 
> <python-list at python.org> wrote:
>
>     On 18/02/2023 15:29, Thomas Passin wrote:
>     > On 2/18/2023 5:38 AM, Albert-Jan Roskam wrote:
>     >>     I sometimes use this trick, which I learnt from a book by
>     Martelli.
>     >>     Instead of try/except, membership testing with "in"
>     >> (__contains__) might
>     >>     be faster. Probably "depends". Matter of measuring.
>     >>     def somefunc(arg, _cache={}):
>     >>         if len(_cache) > 10 ** 5:
>     >>             _cache.pop()
>     >>         try:
>     >>             return _cache[arg]
>     >>         except KeyError:
>     >>             result = expensivefunc(arg)
>     >>             _cache[arg] = result
>     >>             return result
>     >>     Albert-Jan
>     >
>     > _cache.get(arg) should be a little faster and use slightly fewer
>     > resources than the try/except.
>     >
>     Provided that you can provide a default value to get() which will
>     never
>     be a genuine "result".
>
>
> =====
>
> This might be better than None:
> _cache.get(arg, Ellipsis)
>
>
A common strategy is to have a dedicated sentinel object.  E.g. (untested):
IMPOSSIBLE_RESULT = object()
...
         if _cache.get(arg, IMPOSSIBLE_RESULT) == IMPOSSIBLE_RESULT:
             # arg was not in the cache
             ...