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Since 9cd1036455e7, os.stat() on Windows fills the st_ino member;
it could fill st_dev just as easily;
then os.path.samefile() could be shared with the posix implementation.

I created/assigned #10646 to myself for other samefile issues - I can cover this as well unless someone beats me to it.

Here's a patch that fills st_dev, and while we're at it st_rdev (which is the same value).
I've moved the implementation of samefile/sameopenfile/samestat from Lib/posixpath.py over to Lib/genericpath.py and then removed the implementation from Lib/ntpath.py, so those functions are now common. The same goes for tests - I've rearranged tests towards test_genericpath. I also removed _getfileinformation from Modules/posixmodule.c because it's no longer being used.

What about macpath? I think test_macpath will fail.

Why do you think that? I don't have a mac so I can't test it.

Here is an updated patch addressing the sameopenfile that remained in Lib/ntpath.py, thanks to Sehriy's comment on the review.

Hmm. test_macpath will now run the tests for samefile etc. macpath doesn't contain os.path.samefile. But in Modules/posixmodule.c I don't see any special cases for classic MacOS. Actually, I don't know how tests behave on classic MacOS.
Please update the documentation (availability, versionchanged).

On other hand, you should add "samefile", "sameopenfile" and "samestat" back to __all__ in posixpath and ntpath. __all__ is a list of exported names, not a list of defined names.

Docs and the __all__ changes in V3 patch.

New changeset 189b21f9bc0c by Brian Curtin in branch 'default':
Fix #11939. Set st_dev attribute on Windows to simplify os.path.samefile.
http://hg.python.org/cpython/rev/189b21f9bc0c 

New changeset 82531b78b719 by Brian Curtin in branch 'default':
Add NEWS entry for fixing #11939
http://hg.python.org/cpython/rev/82531b78b719 

Thanks for the reviews.

New changeset 9e980454b85e by Brian Curtin in branch 'default':
Add tests for Issue #10646.
http://hg.python.org/cpython/rev/9e980454b85e 

Try on Windows:
>>> import os
>>> from os.path import *
>>> samestat(os.stat('.'), os.stat('.'))
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
NameError: name 'samestat' is not defined

Separate issue. Fixed in #16788.

New changeset 3738d270c54a by Brian Curtin in branch 'default':
st_dev/st_rdev should be unsigned long as dwVolumeSerialNumber, which it is set to, is a DWORD. This was fixed in #11939 and the overflow was mentioned in #10657 and seen by me on some machines.
http://hg.python.org/cpython/rev/3738d270c54a 

> New changeset 3738d270c54a by Brian Curtin in branch 'default':
> st_dev/st_rdev should be unsigned long as dwVolumeSerialNumber, which it is
> set to, is a DWORD. This was fixed in #11939 and the overflow was
> mentioned in #10657 and seen by me on some machines.
> http://hg.python.org/cpython/rev/3738d270c54a
But than st_dev used as long in _pystat_fromstructstat(). Perhaps you should 
check if st_dev is negative and then select 
PyLong_FromLong/PyLong_FromLongLong or 
PyLong_FromUnsignedLong/PyLong_FromUnsignedLongLong.

New changeset 61bada808b34 by Brian Curtin in branch 'default':
Set st_dev on Windows as unsigned long to match its DWORD type, related to the change to fix #11939.
http://hg.python.org/cpython/rev/61bada808b34 

Now the code is wrong on non-Windows without PY_LONG_LONG and with signed st_dev.

Backed out the changeset. If you have a solution, feel free to fix it.