Re: [Python-Dev] PEP 428: Pathlib -> stat caching

On 17 Sep 2013 06:45, "Antoine Pitrou" wrote:

On Mon, 16 Sep 2013 16:14:43 -0400 "R. David Murray" wrote:

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On Mon, 16 Sep 2013 15:48:54 -0400, Brett Cannon

wrote:

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On Mon, Sep 16, 2013 at 3:45 PM, Antoine Pitrou wrote:

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So I would like to propose the following API change:

- Path.stat() (and stat-accessing methods such as get_mtime()...) returns an uncached stat object by default

- Path.cache_stat() can be called to return the stat() *and* cache it for future use, such that any future call to stat(), cache_stat() or a stat-accessing function reuses that cached stat

In other words, only if you use cache_stat() at least once is the stat() value cached and reused by the Path object. (also, it's a per-Path decision)

Any reason why stat() can't get a keyword-only cached=True argument instead? Or have stat() never cache() but stat_cache() always so that people can choose if they want fresh or cached based on API and not whether some library happened to make a decision for them?

Well, we tend to avoid single boolean arguments in favor of differently named functions.

But here is an alternate API: expose the state by having a 'cache_stat' attribute of the Path that is 'False' by default but can be set 'True'.

Thanks for the suggestion, that's a possibility too.

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It could also (or only?) be set via an optional constructor argument.

That's impractical if you get the Path object from a library call.

Given that this is a behavioural state change, I think asking for a possibly *new* path with caching enabled in that case would be a good way to go. If we treat path objects as effectively immutable (aside from the optional internal stat cache), then checking in __new__ if a passed in path object already has the appropriate caching status and returning it directly if so, but otherwise creating a new path object with the cache setting changed would avoid having libraries potentially alter the behaviour of applications' path objects and vice-versa. In effect, the unique "identity" of a path would be a triple representing the type, the filesystem path and whether or not it cached stat results internally. If you wanted to change any of those, you would have to create a new object. Cheers, Nick.

Regards

Antoine.

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