gh-74453: Deprecate os.path.commonprefix

Doc/deprecations/pending-removal-in-future.rst

@@ -78,6 +78,9 @@ 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.
+
 * :class:`!pydoc.ErrorDuringImport`: A tuple value for *exc_info* parameter is
   deprecated, use an exception instance.
 

Doc/library/os.path.rst

@@ -120,6 +120,9 @@ 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.
+
 
 .. function:: dirname(path, /)
 

Lib/genericpath.py

@@ -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

Lib/posixpath.py

@@ -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]))
 
         rel_list = [pardir] * (len(start_list)-i) + path_list[i:]
         if not rel_list:

Lib/test/test_genericpath.py

@@ -34,71 +34,72 @@ def test_no_argument(self):
                                 .format(self.pathmodule.__name__, attr))
 
     def test_commonprefix(self):
-        commonprefix = self.pathmodule.commonprefix
-        self.assertEqual(
-            commonprefix([]),
-            ""
-        )
-        self.assertEqual(
-            commonprefix(["/home/swenson/spam", "/home/swen/spam"]),
-            "/home/swen"
-        )
-        self.assertEqual(
-            commonprefix(["/home/swen/spam", "/home/swen/eggs"]),
-            "/home/swen/"
-        )
-        self.assertEqual(
-            commonprefix(["/home/swen/spam", "/home/swen/spam"]),
-            "/home/swen/spam"
-        )
-        self.assertEqual(
-            commonprefix(["home:swenson:spam", "home:swen:spam"]),
-            "home:swen"
-        )
-        self.assertEqual(
-            commonprefix([":home:swen:spam", ":home:swen:eggs"]),
-            ":home:swen:"
-        )
-        self.assertEqual(
-            commonprefix([":home:swen:spam", ":home:swen:spam"]),
-            ":home:swen:spam"
-        )
-
-        self.assertEqual(
-            commonprefix([b"/home/swenson/spam", b"/home/swen/spam"]),
-            b"/home/swen"
-        )
-        self.assertEqual(
-            commonprefix([b"/home/swen/spam", b"/home/swen/eggs"]),
-            b"/home/swen/"
-        )
-        self.assertEqual(
-            commonprefix([b"/home/swen/spam", b"/home/swen/spam"]),
-            b"/home/swen/spam"
-        )
-        self.assertEqual(
-            commonprefix([b"home:swenson:spam", b"home:swen:spam"]),
-            b"home:swen"
-        )
-        self.assertEqual(
-            commonprefix([b":home:swen:spam", b":home:swen:eggs"]),
-            b":home:swen:"
-        )
-        self.assertEqual(
-            commonprefix([b":home:swen:spam", b":home:swen:spam"]),
-            b":home:swen:spam"
-        )
-
-        testlist = ['', 'abc', 'Xbcd', 'Xb', 'XY', 'abcd',
-                    'aXc', 'abd', 'ab', 'aX', 'abcX']
-        for s1 in testlist:
-            for s2 in testlist:
-                p = commonprefix([s1, s2])
-                self.assertStartsWith(s1, p)
-                self.assertStartsWith(s2, p)
-                if s1 != s2:
-                    n = len(p)
-                    self.assertNotEqual(s1[n:n+1], s2[n:n+1])
+        with warnings_helper.check_warnings((".*commonpath().*", DeprecationWarning)):
+            commonprefix = self.pathmodule.commonprefix
+            self.assertEqual(
+                commonprefix([]),
+                ""
+            )
+            self.assertEqual(
+                commonprefix(["/home/swenson/spam", "/home/swen/spam"]),
+                "/home/swen"
+            )
+            self.assertEqual(
+                commonprefix(["/home/swen/spam", "/home/swen/eggs"]),
+                "/home/swen/"
+            )
+            self.assertEqual(
+                commonprefix(["/home/swen/spam", "/home/swen/spam"]),
+                "/home/swen/spam"
+            )
+            self.assertEqual(
+                commonprefix(["home:swenson:spam", "home:swen:spam"]),
+                "home:swen"
+            )
+            self.assertEqual(
+                commonprefix([":home:swen:spam", ":home:swen:eggs"]),
+                ":home:swen:"
+            )
+            self.assertEqual(
+                commonprefix([":home:swen:spam", ":home:swen:spam"]),
+                ":home:swen:spam"
+            )
+
+            self.assertEqual(
+                commonprefix([b"/home/swenson/spam", b"/home/swen/spam"]),
+                b"/home/swen"
+            )
+            self.assertEqual(
+                commonprefix([b"/home/swen/spam", b"/home/swen/eggs"]),
+                b"/home/swen/"
+            )
+            self.assertEqual(
+                commonprefix([b"/home/swen/spam", b"/home/swen/spam"]),
+                b"/home/swen/spam"
+            )
+            self.assertEqual(
+                commonprefix([b"home:swenson:spam", b"home:swen:spam"]),
+                b"home:swen"
+            )
+            self.assertEqual(
+                commonprefix([b":home:swen:spam", b":home:swen:eggs"]),
+                b":home:swen:"
+            )
+            self.assertEqual(
+                commonprefix([b":home:swen:spam", b":home:swen:spam"]),
+                b":home:swen:spam"
+            )
+
+            testlist = ['', 'abc', 'Xbcd', 'Xb', 'XY', 'abcd',
+                        'aXc', 'abd', 'ab', 'aX', 'abcX']
+            for s1 in testlist:
+                for s2 in testlist:
+                    p = commonprefix([s1, s2])
+                    self.assertStartsWith(s1, p)
+                    self.assertStartsWith(s2, p)
+                    if s1 != s2:
+                        n = len(p)
+                        self.assertNotEqual(s1[n:n+1], s2[n:n+1])
 
     def test_getsize(self):
         filename = os_helper.TESTFN
@@ -606,8 +607,9 @@ 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)

Lib/test/test_ntpath.py

@@ -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
@@ -298,12 +299,13 @@ def test_isabs(self):
         tester('ntpath.isabs("\\\\.\\C:")', 1)
 
     def test_commonprefix(self):
-        tester('ntpath.commonprefix(["/home/swenson/spam", "/home/swen/spam"])',
-               "/home/swen")
-        tester('ntpath.commonprefix(["\\home\\swen\\spam", "\\home\\swen\\eggs"])',
-               "\\home\\swen\\")
-        tester('ntpath.commonprefix(["/home/swen/spam", "/home/swen/spam"])',
-               "/home/swen/spam")
+        with warnings_helper.check_warnings((".*commonpath().*", DeprecationWarning)):
+            tester('ntpath.commonprefix(["/home/swenson/spam", "/home/swen/spam"])',
+                   "/home/swen")
+            tester('ntpath.commonprefix(["\\home\\swen\\spam", "\\home\\swen\\eggs"])',
+                   "\\home\\swen\\")
+            tester('ntpath.commonprefix(["/home/swen/spam", "/home/swen/spam"])',
+                   "/home/swen/spam")
 
     def test_join(self):
         tester('ntpath.join("")', '')

Lib/unittest/util.py

@@ -1,7 +1,6 @@
 """Various utility functions."""
 
 from collections import namedtuple, Counter
-from os.path import commonprefix
 
 __unittest = True
 
@@ -21,13 +20,25 @@ def _shorten(s, prefixlen, suffixlen):
         s = '%s[%d chars]%s' % (s[:prefixlen], skip, s[len(s) - suffixlen:])
     return s
 
+def _commonprefix(m, /):
+    if not m:
+        return ""
+    m = sorted(m)
+    prefix = m[0]
+    for item in m[1:]:
+        for i in range(len(prefix)):
+            if item[i] != prefix[i]:
+                prefix = prefix[:i]
+                break
+    return prefix
+
 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 = _commonprefix(args)
     prefixlen = len(prefix)
 
     common_len = _MAX_LENGTH - \

Misc/NEWS.d/next/Library/2026-02-02-12-09-38.gh-issue-74453.19h4Z5.rst

@@ -0,0 +1,2 @@
+Deprecate :func:`os.path.commonprefix` in favor of
+:func:`os.path.commonpath` for path segment prefixes.