[Python-ideas] Allow manual creation of DirEntry objects

[Guido van Rossum]

Brendan,

The conclusion is that you should just file a bug asking for a working
constructor -- or upload a patch if you want to.

--Guido

On Wed, Aug 17, 2016 at 12:18 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 17 August 2016 at 09:56, Victor Stinner <victor.stinner at gmail.com>
> wrote:
> > 2016-08-17 1:50 GMT+02:00 Guido van Rossum <guido at python.org>:
> >> We could expose the class with a
> >> constructor that always fails (the C code could construct instances
> through
> >> a backdoor).
> >
> > Oh, in fact you cannot create an instance of os.DirEntry, it has no
> > (Python) constructor:
> >
> > $ ./python
> > Python 3.6.0a4+ (default:e615718a6455+, Aug 17 2016, 00:12:17)
> >>>> import os
> >>>> os.DirEntry(1)
> > Traceback (most recent call last):
> >   File "<stdin>", line 1, in <module>
> > TypeError: cannot create 'posix.DirEntry' instances
> >
> > Only os.scandir() can produce such objects.
> >
> > The question is still if it makes sense to allow to create DirEntry
> > objects in Python :-)
>
> I think it does, as it isn't really any different from someone calling
> the stat() method on a DirEntry instance created by os.scandir(). It
> also prevents folks attempting things like:
>
>     def slow_constructor(dirname, entryname):
>         for entry in os.scandir(dirname):
>             if entry.name == entryname:
>                 entry.stat()
>                 return entry
>
> Allowing DirEntry construction from Python further gives us a
> straightforward answer to the "stat caching" question: "just use
> os.DirEntry instances and call stat() to make the snapshot"
>
> If folks ask why os.DirEntry caches results when pathlib.Path doesn't,
> we have the answer that cache invalidation is a hard problem, and
> hence we consider it useful in the lower level interface that is
> optimised for speed, but problematic in the higher level one that is
> more focused on cross-platform correctness of filesystem interactions.
>
> I don't know whether it would make sense to allow a pre-existing stat
> result to be based to DirEntry, but it does seem like it might be
> useful for adapting existing stat-based backend APIs to a more user
> friendly DirEntry based front end API.
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
>



-- 
--Guido van Rossum (python.org/~guido)
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