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

Ben Hoyt benhoyt at gmail.com
Sat Jun 28 21:48:03 CEST 2014

>> 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

> 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

> 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.


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