[Python-Dev] new LRU cache API in Py3.2

[Jason R. Coombs]

I see now that my previous reply went only to Stefan, so I'm re-submitting,
this time to the list.

> -----Original Message-----
> From: Stefan Behnel
> Sent: Saturday, 04 September, 2010 04:29
>
> What about adding an intermediate namespace called "cache", so that 
> the new operations are available like this:
> 
>      print get_phone_number.cache.hits
>      get_phone_number.cache.clear()

I agree. While the function-based implementation is highly efficient, the
pure use of functions has the counter-Pythonic effect of obfuscating the
internal state (the same way the 'private' keyword does in Java). A
class-based implementation would be capable of having its state introspected
and could easily be extended. While the functional implementation is a
powerful construct, it fails to generalize well. IMHO, a stdlib
implementation should err on the side of transparency and extensibility over
performance.

That said, I've adapted Hettinger's Python 2.5 implementation to a
class-based implementation. I've tried to keep the performance optimizations
in place, but instead of instrumenting the wrapped method with lots of
cache_* functions, I simply attach the cache object itself, which then
provides the interface suggested by Stefan. This technique allows access to
the cache object and all of its internal state, so it's also possible to do
things like:

    get_phone_number.cache.maxsize += 100

or

    if get_phone_number.cache.store:
        do_something_interesting()

These techniques are nearly impossible in the functional implementation, as
the state is buried in the locals() of the nested functions.

I'm most grateful to Raymond for contributing this to Python; On many
occasions, I've used the ActiveState recipes for simple caches, but in
almost every case, I've had to adapt the implementation to provide more
transparency. I'd prefer to not have to do the same with the stdlib.

Regards,
Jason R. Coombs

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