[Python-Dev] PEP 471 -- os.scandir() function -- a better and faster directory iterator

[Gregory P. Smith]

On Jun 28, 2014 12:49 PM, "Ben Hoyt" <benhoyt at gmail.com> wrote:
>
> >> But the underlying system calls -- ``FindFirstFile`` /
> >> ``FindNextFile`` on Windows and ``readdir`` on Linux and OS X --
> >
> > What about FreeBSD, OpenBSD, NetBSD, Solaris, etc. They don't provide
readdir?
>
> I guess it'd be better to say "Windows" and "Unix-based OSs"
> throughout the PEP? Because all of these (including Mac OS X) are
> Unix-based.

No, Just say POSIX.

>
> > It looks like the WIN32_FIND_DATA has a dwFileAttributes field. So we
> > should mimic stat_result recent addition: the new
> > stat_result.file_attributes field. Add DirEntry.file_attributes which
> > would only be available on Windows.
> >
> > The Windows structure also contains
> >
> >   FILETIME ftCreationTime;
> >   FILETIME ftLastAccessTime;
> >   FILETIME ftLastWriteTime;
> >   DWORD    nFileSizeHigh;
> >   DWORD    nFileSizeLow;
> >
> > It would be nice to expose them as well. I'm  no more surprised that
> > the exact API is different depending on the OS for functions of the os
> > module.
>
> I think you've misunderstood how DirEntry.lstat() works on Windows --
> it's basically a no-op, as Windows returns the full stat information
> with the original FindFirst/FindNext OS calls. This is fairly explict
> in the PEP, but I'm sure I could make it clearer:
>
>     DirEntry.lstat(): "like os.lstat(), but requires no system calls on
Windows
>
> So you can already get the dwFileAttributes for free by saying
> entry.lstat().st_file_attributes. You can also get all the other
> fields you mentioned for free via .lstat() with no additional OS calls
> on Windows, for example: entry.lstat().st_size.
>
> Feel free to suggest changes to the PEP or scandir docs if this isn't
> clear. Note that is_dir()/is_file()/is_symlink() are free on all
> systems, but .lstat() is only free on Windows.
>
> > Does your implementation uses a free list to avoid the cost of memory
> > allocation? A short free list of 10 or maybe just 1 may help. The free
> > list may be stored directly in the generator object.
>
> No, it doesn't. I might add this to the PEP under "possible
> improvements". However, I think the speed increase by removing the
> extra OS call and/or disk seek is going to be way more than memory
> allocation improvements, so I'm not sure this would be worth it.
>
> > Does it support also bytes filenames on UNIX?
>
> > Python now supports undecodable filenames thanks to the PEP 383
> > (surrogateescape). I prefer to use the same type for filenames on
> > Linux and Windows, so Unicode is better. But some users might prefer
> > bytes for other reasons.
>
> I forget exactly now what my scandir module does, but for os.scandir()
> I think this should behave exactly like os.listdir() does for
> Unicode/bytes filenames.
>
> > Crazy idea: would it be possible to "convert" a DirEntry object to a
> > pathlib.Path object without losing the cache? I guess that
> > pathlib.Path expects a full  stat_result object.
>
> The main problem is that pathlib.Path objects explicitly don't cache
> stat info (and Guido doesn't want them to, for good reason I think).
> There's a thread on python-dev about this earlier. I'll add it to a
> "Rejected ideas" section.
>
> > I don't understand how you can build a full lstat() result without
> > really calling stat. I see that WIN32_FIND_DATA contains the size, but
> > here you call lstat().
>
> See above.
>
> > Do you plan to continue to maintain your module for Python < 3.5, but
> > upgrade your module for the final PEP?
>
> Yes, I intend to maintain the standalone scandir module for 2.6 <=
> Python < 3.5, at least for a good while. For integration into the
> Python 3.5 stdlib, the implementation will be integrated into
> posixmodule.c, of course.
>
> >> Should there be a way to access the full path?
> >> ----------------------------------------------
> >>
> >> Should ``DirEntry``'s have a way to get the full path without using
> >> ``os.path.join(path, entry.name)``? This is a pretty common pattern,
> >> and it may be useful to add pathlib-like ``str(entry)`` functionality.
> >> This functionality has also been requested in `issue 13`_ on GitHub.
> >>
> >> .. _`issue 13`: https://github.com/benhoyt/scandir/issues/13
> >
> > I think that it would be very convinient to store the directory name
> > in the DirEntry. It should be light, it's just a reference.
> >
> > And provide a fullname() name which would just return
> > os.path.join(path, entry.name) without trying to resolve path to get
> > an absolute path.
>
> Yeah, fair suggestion. I'm still slightly on the fence about this, but
> I think an explicit fullname() is a good suggestion. Ideally I think
> it'd be better to mimic pathlib.Path.__str__() which is kind of the
> equivalent of fullname(). But how does pathlib deal with unicode/bytes
> issues if it's the str function which has to return a str object? Or
> at least, it'd be very weird if __str__() returned bytes. But I think
> it'd need to if you passed bytes into scandir(). Do others have
> thoughts?
>
> > Would it be hard to implement the wildcard feature on UNIX to compare
> > performances of scandir('*.jpg') with and without the wildcard built
> > in os.scandir?
>
> It's a good idea, the problem with this is that the Windows wildcard
> implementation has a bunch of crazy edge cases where *.ext will catch
> more things than just a simple regex/glob. This was discussed on
> python-dev or python-ideas previously, so I'll dig it up and add to a
> Rejected Ideas section. In any case, this could be added later if
> there's a way to iron out the Windows quirks.
>
> -Ben
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