diff -r 4a9dbd9b152e Doc/library/os.path.rst
--- a/Doc/library/os.path.rst	Mon Nov 05 00:56:02 2012 +1000
+++ b/Doc/library/os.path.rst	Sun Nov 04 11:05:17 2012 -0500
@@ -60,6 +60,18 @@
    of all paths in  *list*.  If *list* is empty, return the empty string (``''``).
    Note that this may return invalid paths because it works a character at a time.
 
+.. function:: commonpath(paths)
+
+   Return the longest common sub-path of each pathname in the sequence
+   *paths*.  The paths must be absolute, otherwise a ValueError is raised.
+   If *paths* is empty, return ``None``.  If each path ends with a path
+   separator, the returned sub-path will also end with a separator, otherwise
+   it will not.  Unlike :func:`commonprefix`, this returns a valid path.
+
+   Availability:  Unix.
+
+   .. versionadded:: 3.4
+
 
 .. function:: dirname(path)
 
diff -r 4a9dbd9b152e Lib/posixpath.py
--- a/Lib/posixpath.py	Mon Nov 05 00:56:02 2012 +1000
+++ b/Lib/posixpath.py	Sun Nov 04 11:05:17 2012 -0500
@@ -22,7 +22,8 @@
            "ismount", "expanduser","expandvars","normpath","abspath",
            "samefile","sameopenfile","samestat",
            "curdir","pardir","sep","pathsep","defpath","altsep","extsep",
-           "devnull","realpath","supports_unicode_filenames","relpath"]
+           "devnull","realpath","supports_unicode_filenames","relpath",
+           "commonpath"]
 
 # Strings representing various path-related bits and pieces.
 # These are primarily for export; internally, they are hardcoded.
@@ -467,3 +468,36 @@
     if not rel_list:
         return curdir
     return join(*rel_list)
+
+
+def commonpath(paths):
+    """Given a sequence of absolute path names, returns the longest common
+    sub-path."""
+
+    if not paths:
+        return None
+
+    if not all(isabs(p) for p in paths):
+        raise ValueError('All paths must be absolute')
+
+    sep = _get_sep(paths[0])
+
+    try:
+        split_paths = [path.split(sep) for path in paths]
+    except TypeError:
+        valid_types = all(isinstance(p, (str, bytes, bytearray))
+                          for p in paths)
+        if valid_types:
+            # Must have a mixture of text and binary data
+            raise TypeError("Can't mix strings and bytes in path "
+                            "components.") from None
+        raise
+
+    zipped_components = list(zip(*split_paths))
+    for i, cs in enumerate(zipped_components):
+        first = cs[0]
+        if not all(c == first for c in cs):
+            i = i - 1   # last matching index
+            break
+    common = [cs[0] for cs in zipped_components[:i+1]]
+    return sep + join(*common)
diff -r 4a9dbd9b152e Lib/test/test_posixpath.py
--- a/Lib/test/test_posixpath.py	Mon Nov 05 00:56:02 2012 +1000
+++ b/Lib/test/test_posixpath.py	Sun Nov 04 11:05:17 2012 -0500
@@ -523,6 +523,31 @@
         with open(fname, "wb") as a, open(fname, "wb") as b:
             self.assertTrue(posixpath.sameopenfile(a.fileno(), b.fileno()))
 
+    def test_commonpath(self):
+
+        self.assertIsNone(posixpath.commonpath([]))
+        self.assertEqual(posixpath.commonpath(['/usr/local']), '/usr/local')
+        self.assertEqual(posixpath.commonpath(['/usr/local', '/usr/local']),
+                         '/usr/local')
+        self.assertEqual(posixpath.commonpath(['/usr/local/', '/usr/local']),
+                         '/usr/local')
+        self.assertEqual(posixpath.commonpath(['/usr/local/', '/usr/local/']),
+                         '/usr/local/')
+        self.assertEqual(posixpath.commonpath(['/', '/dev']), '/')
+        self.assertEqual(
+            posixpath.commonpath(['/usr/lib/', '/usr/lib/python3']),
+            '/usr/lib')
+        self.assertEqual(
+            posixpath.commonpath(['/usr/lib/', '/usr/lib/', '/dev/']), '/')
+        self.assertEqual(
+            posixpath.commonpath([b'/usr/lib/', b'/usr/lib/python3']),
+            b'/usr/lib')
+        self.assertEqual(posixpath.commonpath(['/usr/lib/', '/usr/lib64/']),
+                         '/usr')
+        self.assertRaises(TypeError, posixpath.commonpath,
+                          [b'/usr/lib/', '/usr/lib/python3'])
+        self.assertRaises(ValueError, posixpath.commonpath, ['spam', 'alot'])
+
 
 class PosixCommonTest(test_genericpath.CommonTest):
     pathmodule = posixpath