Use scandir() to speed up pathlib globbing #70220

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1. gvanrossum commented on Jan 6, 2016

   

   gvanrossum

   on Jan 6, 2016

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   The globbing functionality in pathlib (Path.glob() and Path.rglob()) might benefit from using the new optimized os.scandir() interface. It currently just uses os.listdir(). The Path.iterdir() method might also benefit (though less so).

   There's also a sideways connection with http://bugs.python.org/issue26031 (adding an optional stat cache) -- the cache could possibly keep the DirEntry objects and use their (hopefully cached) attributes. This is more speculative though (and what if the platform's DirEntry doesn't cache?)
2. gvanrossum

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   performancePerformance or resource usagePerformance or resource usage

   on Jan 6, 2016
3. vstinner commented on Jan 6, 2016

   

   vstinner

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   Related issue: issue bpo-25596 "regular files handled as directories in the glob module".
4. ethanfurman commented on Jan 6, 2016

   

   ethanfurman

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   As I recall, if the platform's DirEntry doesn't provide the cacheable attributes when first called, those attributes will be looked up (and cached) on first access.
5. vstinner commented on Jan 6, 2016

   

   vstinner

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   As I recall, if the platform's DirEntry doesn't provide the cacheable attributes when first called, those attributes will be looked up (and cached) on first access.

   scandir() is not magic. It simply provides info given by the OS: see readdir() on UNIX and FindFirstFile()/FindNextFile() on Windows.

   DirEntry calls os.stat() if needed, but it caches the result.

   DirEntry doc tries to explain when syscalls or required or not, depending on the requested information and the platform:
   https://docs.python.org/dev/library/os.html#os.DirEntry
6. gvanrossum commented on Jan 6, 2016

   

   gvanrossum

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   The DirEntry docs say for most methods "In most cases, no system call is
   required" which is pretty non-committal. :-( The only firm promise is for
   inode(), which is pretty useless.
7. benhoyt commented on Jan 7, 2016

   

   benhoytmannequin

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   Guido, it's true that in almost all cases you get the speedup (no system call), and it's very much worth using. But the idea with the docs being non-committal is because being specific would make the docs fairly complex. I believe it's as follows for is_file/is_dir/is_symlink:

   • no system call required on Windows or Unix if the entry is not a symlink
   • unless you're on Unix with some different file system (maybe a network FS?) where d_type is DT_UNKNOWN
   • some other edge case which I've probably forgotten :-)

   Do you think the docs should try to make this more specific?
8. gvanrossum commented on Jan 7, 2016

   

   gvanrossum

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   Ben, I think it's worth calling out what the rules are around symlinks. I'm
   guessing the info that is initially present is a subset of lstat(), so if
   that indicates it's a symlink, is_dir() and is_file() will need a stat()
   call, *unless* follow_symlinks is False.

   Another question: for symlinks, there are two different possible stat
   results: one for stat() and one for lstat(). Are these both cached
   separately? Or is only one of them? (Experimentally, they are either both
   cached or the cache remembers the follow_symlinks flag and re-fetches the
   other result.)

   Related, "this method always requires a system call", that remark seems to
   disregard the cache.

   I'd be happy to review a doc update patch if you make one.
9. vstinner commented on Jan 7, 2016

   

   vstinner

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   "Another question: for symlinks, there are two different possible stat
   results: one for stat() and one for lstat(). Are these both cached
   separately?"

   Hopefully, both are cached. It's directly the result of stat() and
   stat(follow_symlinks=False) which are cached (so a os.stat_result
   object).
10. serhiy-storchaka

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    stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

    on Jan 7, 2016
11. serhiy-storchaka commented on Jan 11, 2016

    

    serhiy-storchaka

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    Proposed minimal patch implements globbing in pathlib using os.scandir(). Here are results of microbenchmarks:

    $ ./python -m timeit -s "from pathlib import Path; p = Path()" -- "list(p.glob('**/*'))"
    Unpatched: 598 msec per loop
    Patched:   372 msec per loop
    
    $ ./python -m timeit -s "from pathlib import Path; p = Path('/usr/')" -- "list(p.glob('lib*/**/*'))"
    Unpatched: 1.33 sec per loop
    Patched:   804 msec per loop
    
    $ ./python -m timeit -s "from pathlib import Path; p = Path('/usr/')" -- "list(p.glob('lib*/**/'))"
    Unpatched: 750 msec per loop
    Patched:   180 msec per loop

    See msg257954 in bpo-25596 for comparison with the glob module.
12. serhiy-storchaka

    self-assigned this

    on Jan 11, 2016
13. benhoyt commented on Jan 31, 2016

    

    benhoytmannequin

    on Jan 31, 2016

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    Guido, I've made some tweaks and improvements to the DirEntry docs here: http://bugs.python.org/issue26248 -- the idea is to fix the issues you mentioned to clarify when system calls are required with symlinks, mentioning that the results are cached separately for follow_symlinks True and False, etc.
14. python-dev commented on Sep 7, 2016

    

    python-devmannequin

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    New changeset 927665c4aaab by Serhiy Storchaka in branch 'default':
    Issue bpo-26032: Optimized globbing in pathlib by using os.scandir(); it is now
    https://hg.python.org/cpython/rev/927665c4aaab
15. serhiy-storchaka

    closed this as completedon Sep 7, 2016
16. ezio-melotti

    transferred this issue fromon Apr 10, 2022