[Python-Dev] PEP 428: Pathlib -> stat caching (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Tue Sep 17 01:15:26 CEST 2013

On 17 Sep 2013 06:45, "Antoine Pitrou" <solipsis at pitrou.net> wrote:

On Mon, 16 Sep 2013 16:14:43 -0400 "R. David Murray" <rdmurray at bitdance.com> wrote: > On Mon, 16 Sep 2013 15:48:54 -0400, Brett Cannon <brett at python.org> wrote: > > On Mon, Sep 16, 2013 at 3:45 PM, Antoine Pitrou <solipsis at pitrou.net> wrote: > > > So I would like to propose the following API change: > > > > > > - Path.stat() (and stat-accessing methods such as getmtime()...) > > > returns an uncached stat object by default > > > > > > - Path.cachestat() can be called to return the stat() and cache it > > > for future use, such that any future call to stat(), cachestat() or > > > a stat-accessing function reuses that cached stat > > > > > > In other words, only if you use cachestat() 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 statcache() 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 'cachestat' > attribute of the Path that is 'False' by default but can be set 'True'. Thanks for the suggestion, that's a possibility too. > 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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