Index: Doc/library/os.path.rst
===================================================================
--- Doc/library/os.path.rst	(revision 67929)
+++ Doc/library/os.path.rst	(working copy)
@@ -53,6 +53,16 @@
    Note that this may return invalid paths because it works a character at a time.
 
 
+.. function:: commonpathprefix(list, separator=None)
+
+   Return the longest path prefix (taken component-by-component) that is a
+   prefix of all paths in *list*.  If *list* is empty, return the empty
+   string (``''``).  Unlike :func:`commonprefix`, this will only return the
+   empty string or complete paths.  The optional second argument defaults to
+   os.sep on platforms other than Windows and OS/2.  On those platforms it
+   defaults to either os.sep or os.altsep.
+
+
 .. function:: dirname(path)
 
    Return the directory name of pathname *path*.  This is the first half of the
Index: Lib/genericpath.py
===================================================================
--- Lib/genericpath.py	(revision 67929)
+++ Lib/genericpath.py	(working copy)
@@ -66,7 +66,7 @@
 
 # Return the longest prefix of all list elements.
 def commonprefix(m):
-    "Given a list of pathnames, returns the longest common leading component"
+    "Given a list of pathnames, returns the longest common leading string"
     if not m: return ''
     s1 = min(m)
     s2 = max(m)
@@ -75,6 +75,26 @@
             return s1[:i]
     return s1
 
+# Return the longest prefix of all list elements.
+def commonpathprefix(m, separator=None):
+    """Given a list of pathnames, returns the longest common leading component.
+
+    Unlike commonprefix(), commonpathprefix() considers path elements as
+    delimited by the defined separator (default: os.sep).
+    """
+    if not m:
+        return ''
+    if separator is None:
+        import os
+        separator = os.sep
+    common = []
+    s1 = min(m).split(separator)
+    s2 = max(m).split(separator)
+    while s1 and s2 and s1[0] == s2[0]:
+        common.append(s1[0])
+        del s1[0], s2[0]
+    return separator.join(common)
+
 # Split a path in root and extension.
 # The extension is everything starting at the last dot in the last
 # pathname component; the root is everything before that.
Index: Lib/os2emxpath.py
===================================================================
--- Lib/os2emxpath.py	(revision 67929)
+++ Lib/os2emxpath.py	(working copy)
@@ -9,7 +9,7 @@
 import stat
 from genericpath import *
 from ntpath import (expanduser, expandvars, isabs, islink, splitdrive,
-                    splitext, split)
+                    splitext, split, commonpathprefix)
 
 __all__ = ["normcase","isabs","join","splitdrive","split","splitext",
            "basename","dirname","commonprefix","getsize","getmtime",
Index: Lib/ntpath.py
===================================================================
--- Lib/ntpath.py	(revision 67929)
+++ Lib/ntpath.py	(working copy)
@@ -64,6 +64,26 @@
     else:
         return ':'
 
+# Return the longest prefix of all list elements.
+def commonpathprefix(m, separator=None):
+    """Given a list of pathnames, returns the longest common leading component.
+
+    Unlike commonprefix(), commonpathprefix() considers path elements as
+    delimited by the defined separator (default: os.sep).
+    """
+    if not m:
+        return ''
+    if separator is None:
+        separator = r"\\|/"
+    import re
+    common = []
+    s1 = re.split(separator, min(m))
+    s2 = re.split(separator, max(m))
+    while s1 and s2 and s1[0] == s2[0]:
+        common.append(s1[0])
+        del s1[0], s2[0]
+    return "\\".join(common)
+
 # Normalize the case of a pathname and map slashes to backslashes.
 # Other normalizations (such as optimizing '../' away) are not done
 # (this is done by normpath).
Index: Lib/macpath.py
===================================================================
--- Lib/macpath.py	(revision 67929)
+++ Lib/macpath.py	(working copy)
@@ -34,7 +34,6 @@
 def normcase(path):
     return path.lower()
 
-
 def isabs(s):
     """Return true if a path is absolute.
     On the Mac, relative paths begin with a colon,
@@ -45,7 +44,16 @@
     colon = _get_colon(s)
     return colon in s and s[:1] != colon
 
+def commonpathprefix(m, separator=None):
+    """Given a list of pathnames, returns the longest common leading component.
 
+    Unlike commonprefix(), commonpathprefix() considers path elements as
+    delimited by the defined separator (default: os.sep).
+    """
+    if separator is None:
+        separator = sep
+    return genericpath.commonpathprefix(m, separator)
+
 def join(s, *p):
     colon = _get_colon(s)
     path = s
Index: Lib/test/test_ntpath.py
===================================================================
--- Lib/test/test_ntpath.py	(revision 67929)
+++ Lib/test/test_ntpath.py	(working copy)
@@ -73,6 +73,40 @@
         tester('ntpath.isabs("\\foo")', 1)
         tester('ntpath.isabs("\\foo\\bar")', 1)
 
+    def test_commonpathprefix(self):
+        self.assertEqual(ntpath.commonpathprefix([]), "")
+        self.assertEqual(ntpath.commonpathprefix(["/home/swenson/spam",
+                                                  "/home/swen/spam"]),
+                         "\\home")
+        self.assertEqual(ntpath.commonpathprefix(["\\home\\swenson\\spam",
+                                                  "\\home\\swen\\spam"]),
+                         "\\home")
+        self.assertEqual(
+            ntpath.commonpathprefix(["\\home\\swen\\spam",
+                                     "\\home\\swen\\eggs"]),
+            "\\home\\swen"
+        )
+        self.assertEqual(
+            ntpath.commonpathprefix(["\\home\\swen\\spam",
+                                     "\\home\\swen\\spam"]),
+            "\\home\\swen\\spam"
+        )
+        self.assertEqual(
+            ntpath.commonpathprefix(["\\home\\swen\\spam",
+                                     "swen\\spam"]),
+            ""
+        )
+        self.assertEqual(
+            ntpath.commonpathprefix(["swen\\eggs",
+                                     "swen\\spam"]),
+            "swen"
+        )
+        self.assertEqual(
+            ntpath.commonpathprefix(["\\export\\home\\swen",
+                                     "\\etc\\passwd"]),
+            ""
+        )
+
     def test_commonprefix(self):
         tester('ntpath.commonprefix(["/home/swenson/spam", "/home/swen/spam"])',
                "/home/swen")
Index: Lib/test/test_genericpath.py
===================================================================
--- Lib/test/test_genericpath.py	(revision 67929)
+++ Lib/test/test_genericpath.py	(working copy)
@@ -25,7 +25,56 @@
             genericpath.commonprefix(["/home/swen/spam", "/home/swen/spam"]),
             "/home/swen/spam"
         )
+        self.assertEqual(
+            genericpath.commonprefix(["/export/home/swen", "/etc/passwd"]),
+            "/e"
+        )
 
+    def test_commonpathprefix(self):
+        self.assertEqual(
+            genericpath.commonpathprefix([]),
+            ""
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["/home/swenson/spam",
+                                          "/home/swen/spam"]),
+            "/home"
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["/home/swenson/spam",
+                                          "/home/swen/spam"], ":"),
+            ""
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["WhomeWswensonWspam",
+                                          "WhomeWswenWspam"], "W"),
+            "Whome"
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["/home/swen/spam",
+                                          "/home/swen/eggs"]),
+            "/home/swen"
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["/home/swen/spam",
+                                          "/home/swen/spam"]),
+            "/home/swen/spam"
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["/home/swen/spam",
+                                          "swen/spam"]),
+            ""
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["swen/eggs",
+                                          "swen/spam"]),
+            "swen"
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["/export/home/swen", "/etc/passwd"]),
+            ""
+        )
+
     def test_getsize(self):
         f = open(support.TESTFN, "wb")
         try:
Index: Lib/test/test_macpath.py
===================================================================
--- Lib/test/test_macpath.py	(revision 67929)
+++ Lib/test/test_macpath.py	(working copy)
@@ -43,6 +43,37 @@
         self.assert_(commonprefix([b":home:swen:spam", b":home:swen:spam"])
                      == b":home:swen:spam")
 
+    def test_commonpathprefix(self):
+        commonpathprefix = macpath.commonpathprefix
+        self.assertEqual(
+            commonpathprefix([]),
+            ""
+        )
+        self.assertEqual(
+            commonpathprefix([":home:swenson:spam", ":home:swen:spam"]),
+            ":home"
+        )
+        self.assertEqual(
+            commonpathprefix([":home:swen:spam", ":home:swen:eggs"]),
+            ":home:swen"
+        )
+        self.assertEqual(
+            commonpathprefix([":home:swen:spam", ":home:swen:spam"]),
+            ":home:swen:spam"
+        )
+        self.assertEqual(
+            commonpathprefix([":home:swen:spam", "swen:spam"]),
+            ""
+        )
+        self.assertEqual(
+            commonpathprefix(["swen:eggs", "swen:spam"]),
+            "swen"
+        )
+        self.assertEqual(
+            commonpathprefix([":export:home:swen", ":etc:passwd"]),
+            ""
+        )
+
     def test_split(self):
         split = macpath.split
         self.assertEquals(split("foo:bar"),
Index: Lib/test/test_posixpath.py
===================================================================
--- Lib/test/test_posixpath.py	(revision 67929)
+++ Lib/test/test_posixpath.py	(working copy)
@@ -165,6 +165,41 @@
 
         self.assertRaises(TypeError, posixpath.dirname)
 
+    def test_commonpathprefix(self):
+        self.assertEqual(
+            genericpath.commonpathprefix([]),
+            ""
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["/home/swenson/spam",
+                                          "/home/swen/spam"]),
+            "/home"
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["/home/swen/spam",
+                                          "/home/swen/eggs"]),
+            "/home/swen"
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["/home/swen/spam",
+                                          "/home/swen/spam"]),
+            "/home/swen/spam"
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["/home/swen/spam",
+                                          "swen/spam"]),
+            ""
+        )
+        self.assertEqual(
+            genericpath.commonpathprefix(["swen/eggs",
+                                          "swen/spam"]),
+            "swen"
+        )
+        self.assertEqual(
+            genericpath.commonprefix(["/export/home/swen", "/etc/passwd"]),
+            ""
+        )
+
     def test_commonprefix(self):
         self.assertEqual(
             posixpath.commonprefix([]),