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.