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gh-81520: Add documentatoion about unexpected os.path.ismount() behaviour #136058

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gh-81520: Add documentatoion about unexpected os.path.ismount() behaviour #136058

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2fe1c6f
Document os.path.ismount() behaviour with btrfs subvolumes.
bionade24 Jun 28, 2025
d8cee97
os.path.ismount(): Put unexpected behaviour into docstring.
bionade24 Jun 28, 2025
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7 changes: 4 additions & 3 deletions Doc/library/os.path.rst
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Expand Up @@ -298,9 +298,10 @@ the :mod:`glob` module.)
device than *path*, or whether :file:`{path}/..` and *path* point to the same
i-node on the same device --- this should detect mount points for all Unix
and POSIX variants. It is not able to reliably detect bind mounts on the
same filesystem. On Windows, a drive letter root and a share UNC are
always mount points, and for any other path ``GetVolumePathName`` is called
to see if it is different from the input path.
same filesystem. On Linux systems it will return True for btrfs subvolumes,
even if they aren't mountpoints. On Windows, a drive letter root and a share
UNC are always mount points, and for any other path ``GetVolumePathName`` is
called to see if it is different from the input path.

.. versionchanged:: 3.4
Added support for detecting non-root mount points on Windows.
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4 changes: 3 additions & 1 deletion Lib/posixpath.py
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Expand Up @@ -188,7 +188,9 @@ def dirname(p):
# (Does this work for all UNIXes? Is it even guaranteed to work by Posix?)

def ismount(path):
"""Test whether a path is a mount point"""
"""Test whether a path is a mount point. Does not work reliably
with bind mounts on the same filesystems, has false positives for
btrfs subvolumes that aren't a mountpoint."""
try:
s1 = os.lstat(path)
except (OSError, ValueError):
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