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gh-74453: Deprecate os.path.commonprefix #144436

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f381a5e
gh-74453: Deprecate os.path.commonprefix
sethmlarson Feb 3, 2026
e8ca19c
Address review comments
sethmlarson Feb 3, 2026
cc320d1
Add entry to 'pending-removal-in-future'
sethmlarson Feb 3, 2026
879d6ec
Address review comments
sethmlarson Feb 3, 2026
1f2086b
Apply suggestions from code review
sethmlarson Feb 3, 2026
166c39d
Remove 'string.commonprefix()' as an alternative
sethmlarson Feb 3, 2026
f58e68f
Explain why os.path.commonprefix() is being deprecated
sethmlarson Feb 4, 2026
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8 changes: 8 additions & 0 deletions Doc/deprecations/pending-removal-in-future.rst
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Expand Up @@ -78,6 +78,14 @@ although there is currently no date scheduled for their removal.

* :mod:`os`: Calling :func:`os.register_at_fork` in a multi-threaded process.

* :mod:`os.path`: :func:`os.path.commonprefix` is deprecated, use
:func:`os.path.commonpath` for path prefixes. The :func:`os.path.commonprefix`
function is being deprecated due to having a misleading name and module.
The function is not safe to use for path prefixes despite being included in a
module about path manipulation, meaning it is easy to accidentally
introduce path traversal vulnerabilities into Python programs by using this
function.

* :class:`!pydoc.ErrorDuringImport`: A tuple value for *exc_info* parameter is
deprecated, use an exception instance.

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8 changes: 8 additions & 0 deletions Doc/library/os.path.rst
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Expand Up @@ -120,6 +120,14 @@ the :mod:`glob` module.)
.. versionchanged:: 3.6
Accepts a :term:`path-like object`.

.. deprecated:: next
Deprecated in favor of :func:`os.path.commonpath` for path prefixes.
The :func:`os.path.commonprefix` function is being deprecated due to
having a misleading name and module. The function is not safe to use for
path prefixes despite being included in a module about path manipulation,
meaning it is easy to accidentally introduce path traversal
vulnerabilities into Python programs by using this function.


.. function:: dirname(path, /)

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9 changes: 9 additions & 0 deletions Lib/genericpath.py
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Expand Up @@ -105,6 +105,15 @@ def getctime(filename, /):
# Return the longest prefix of all list elements.
def commonprefix(m, /):
"Given a list of pathnames, returns the longest common leading component"
import warnings
warnings.warn('os.path.commonprefix() is deprecated. Use '
'os.path.commonpath() for longest path prefix.',
category=DeprecationWarning,
stacklevel=2)
return _commonprefix(m)

def _commonprefix(m, /):
"Internal implementation of commonprefix()"
if not m: return ''
# Some people pass in a list of pathname parts to operate in an OS-agnostic
# fashion; don't try to translate in that case as that's an abuse of the
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2 changes: 1 addition & 1 deletion Lib/posixpath.py
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Expand Up @@ -542,7 +542,7 @@ def relpath(path, start=None):
start_list = start_tail.split(sep) if start_tail else []
path_list = path_tail.split(sep) if path_tail else []
# Work out how much of the filepath is shared by start and path.
i = len(commonprefix([start_list, path_list]))
i = len(genericpath._commonprefix([start_list, path_list]))
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@vstinner vstinner Feb 5, 2026

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Ah, this call is safe since it pass two lists of strings, and not two strings.


rel_list = [pardir] * (len(start_list)-i) + path_list[i:]
if not rel_list:
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9 changes: 7 additions & 2 deletions Lib/test/test_genericpath.py
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Expand Up @@ -34,6 +34,10 @@ def test_no_argument(self):
.format(self.pathmodule.__name__, attr))

def test_commonprefix(self):
with warnings_helper.check_warnings((".*commonpath().*", DeprecationWarning)):
self.do_test_commonprefix()

def do_test_commonprefix(self):
commonprefix = self.pathmodule.commonprefix
self.assertEqual(
commonprefix([]),
Expand Down Expand Up @@ -606,8 +610,9 @@ def test_path_isdir(self):
self.assertPathEqual(os.path.isdir)

def test_path_commonprefix(self):
self.assertEqual(os.path.commonprefix([self.file_path, self.file_name]),
self.file_name)
with warnings_helper.check_warnings((".*commonpath().*", DeprecationWarning)):
self.assertEqual(os.path.commonprefix([self.file_path, self.file_name]),
self.file_name)

def test_path_getsize(self):
self.assertPathEqual(os.path.getsize)
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5 changes: 5 additions & 0 deletions Lib/test/test_ntpath.py
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Expand Up @@ -10,6 +10,7 @@
from ntpath import ALL_BUT_LAST, ALLOW_MISSING
from test import support
from test.support import os_helper
from test.support import warnings_helper
from test.support.os_helper import FakePath
from test import test_genericpath
from tempfile import TemporaryFile
Expand Down Expand Up @@ -298,6 +299,10 @@ def test_isabs(self):
tester('ntpath.isabs("\\\\.\\C:")', 1)

def test_commonprefix(self):
with warnings_helper.check_warnings((".*commonpath().*", DeprecationWarning)):
self.do_test_commonprefix()

def do_test_commonprefix(self):
tester('ntpath.commonprefix(["/home/swenson/spam", "/home/swen/spam"])',
"/home/swen")
tester('ntpath.commonprefix(["\\home\\swen\\spam", "\\home\\swen\\eggs"])',
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13 changes: 11 additions & 2 deletions Lib/unittest/util.py
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@@ -1,7 +1,6 @@
"""Various utility functions."""

from collections import namedtuple, Counter
from os.path import commonprefix

__unittest = True

Expand All @@ -21,13 +20,23 @@ def _shorten(s, prefixlen, suffixlen):
s = '%s[%d chars]%s' % (s[:prefixlen], skip, s[len(s) - suffixlen:])
return s

def _common_prefix(m):
if not m:
return ""
s1 = min(m)
s2 = max(m)
for i, c in enumerate(s1):
if c != s2[i]:
return s1[:i]
return s1

def _common_shorten_repr(*args):
args = tuple(map(safe_repr, args))
maxlen = max(map(len, args))
if maxlen <= _MAX_LENGTH:
return args

prefix = commonprefix(args)
prefix = _common_prefix(args)
prefixlen = len(prefix)

common_len = _MAX_LENGTH - \
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8 changes: 8 additions & 0 deletions Misc/NEWS.d/next/Library/2026-02-02-12-09-38.gh-issue-74453.19h4Z5.rst
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@@ -0,0 +1,8 @@
Deprecate :func:`os.path.commonprefix` in favor of
:func:`os.path.commonpath` for path segment prefixes.

The :func:`os.path.commonprefix` function is being deprecated due to
having a misleading name and module. The function is not safe to use for
path prefixes despite being included in a module about path manipulation,
meaning it is easy to accidentally introduce path traversal
vulnerabilities into Python programs by using this function.
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