[Python-Dev] Re: os.scandir bug in Windows?

[Eryk Sun]

On 10/15/20, Rob Cliffe via Python-Dev  wrote:
>
> TLDR: In os.scandir directory entries, atime is always a copy of mtime
> rather than the actual access time.

There are inconsistencies in various scenarios between between the
stat info from the directory entry and the stat info from the File
Control Block (FCB) -- the filesystem's in-memory record that's common
to all opens for a file/directory.

The worst case is for an NTFS file with multiple hardlinks, for which
the directory entry information is from the last time the file was
opened using a particular hardlink. The accurate NTFS file information
is in the file's Master File Table (MFT) record, which gets accessed
to populate the FCB and update the particular link when a file is
opened.

If you're looking for file times and file size, the only reliable
information comes from directly opening the file an querying the info
via GetFileInformationByHandle (called by os.stat),
GetFileInformationByHandleEx (FileBasicInfo, FileStandardInfo),
GetFileTime, and GetFileSizeEx.
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[Python-Dev] Re: os.scandir bug in Windows?

[Rob Cliffe via Python-Dev]

How do I do that, please?  I can't see an obvious create option on that 
web page.  Do I need to log in?

Thanks
Rob Cliffe

On 18/10/2020 05:31, Gregory P. Smith wrote:
Could you please file this as an issue on bugs.python.org 
?


Thanks!
-Greg


On Sat, Oct 17, 2020 at 7:25 PM Rob Cliffe via Python-Dev 
mailto:python-dev@python.org>> wrote:



TLDR: In os.scandir directory entries, atime is always a copy of
mtime
rather than the actual access time.

Demo program: Windows 10, Python 3.8.3:

# osscandirtest.py
import time, os
with open('Test', 'w') as f: f.write('Anything\n') # Write to a file
time.sleep(10)
with open('Test', 'r') as f: f.readline() # Read the file
print(os.stat('Test'))
for DirEntry in os.scandir('.'):
 if DirEntry.name == 'Test':
 stat = DirEntry.stat()
 print(f'scandir DirEntry {stat.st_ctime=} {stat.st_mtime=}
{stat.st_atime=}')

Sample output:

os.stat_result(st_mode=33206, st_ino=8162774324687317,
st_dev=2230120362, st_nlink=1, st_uid=0,
st_gid=0, st_size=10, st_atime=1600631381, st_mtime=1600631371,
st_ctime=1600631262)
scandir DirEntry stat.st_ctime=1600631262.951019
stat.st_mtime=1600631371.7062848 stat.st_atime=1600631371.7062848

For os.stat, atime is 10 seconds more than mtime, as would be
expected.
But for os.scandir, atime is a copy of mtime.
ISTM that this is a bug, and in fact recently it stopped me from
using
os.scandir in a program where I needed the access timestamp. No big
deal, but ...
If it is a feature for some reason, presumably it should be
documented.

Best wishes
Rob Cliffe
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[Python-Dev] Re: os.scandir bug in Windows?

[Eric V. Smith]

On 10/18/2020 12:25 PM, Rob Cliffe via Python-Dev wrote:
How do I do that, please?  I can't see an obvious create option on 
that web page.  Do I need to log in?


Yes, you need to log in before you can open an issue. You might need to 
create an account first if you don't have one: it's called "Register" on 
bpo. After you've logged in, there's a Create New button.


Eric



Thanks
Rob Cliffe

On 18/10/2020 05:31, Gregory P. Smith wrote:
Could you please file this as an issue on bugs.python.org 
?


Thanks!
-Greg


On Sat, Oct 17, 2020 at 7:25 PM Rob Cliffe via Python-Dev 
mailto:python-dev@python.org>> wrote:



TLDR: In os.scandir directory entries, atime is always a copy of
mtime
rather than the actual access time.

Demo program: Windows 10, Python 3.8.3:

# osscandirtest.py
import time, os
with open('Test', 'w') as f: f.write('Anything\n') # Write to a file
time.sleep(10)
with open('Test', 'r') as f: f.readline() # Read the file
print(os.stat('Test'))
for DirEntry in os.scandir('.'):
 if DirEntry.name == 'Test':
 stat = DirEntry.stat()
 print(f'scandir DirEntry {stat.st_ctime=} {stat.st_mtime=}
{stat.st_atime=}')

Sample output:

os.stat_result(st_mode=33206, st_ino=8162774324687317,
st_dev=2230120362, st_nlink=1, st_uid=0,
st_gid=0, st_size=10, st_atime=1600631381, st_mtime=1600631371,
st_ctime=1600631262)
scandir DirEntry stat.st_ctime=1600631262.951019
stat.st_mtime=1600631371.7062848 stat.st_atime=1600631371.7062848

For os.stat, atime is 10 seconds more than mtime, as would be
expected.
But for os.scandir, atime is a copy of mtime.
ISTM that this is a bug, and in fact recently it stopped me from
using
os.scandir in a program where I needed the access timestamp. No big
deal, but ...
If it is a feature for some reason, presumably it should be
documented.

Best wishes
Rob Cliffe
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[Python-Dev] Re: Changing Python's string search algorithms

[Greg Ewing]

On 17/10/20 3:26 pm, Tim Peters wrote:

Tal Einat posted earlier that he was keen to try to work up a clear
explanation, and I look forward to that. All the expositions I've
found of this algorithm so far are hard to approach.


Maybe Mathologer or 3blue1brown could be persuaded to help?
They seem to have a knack for making tricky stuff understandable.

--
Greg
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[Python-Dev] Re: Changing Python's string search algorithms

[Raymond Hettinger]

> On Oct 17, 2020, at 2:40 PM, Tim Peters  wrote:
> 
> Still waiting for someone who thinks string search speed is critical
> in their real app to give it a try.  In the absence of that, I endorse
> merging this.

Be bold.  Merge it.   :-)


Raymond
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