patch [ 1105730 ] Faster commonprefix in macpath, ntpath, etc.

Lib/macpath.py

@@ -175,14 +175,14 @@ def lexists(path):
 def commonprefix(m):
     "Given a list of pathnames, returns the longest common leading component"
     if not m: return ''
-    prefix = m[0]
-    for item in m:
-        for i in range(len(prefix)):
-            if prefix[:i+1] != item[:i+1]:
-                prefix = prefix[:i]
-                if i == 0: return ''
-                break
-    return prefix
+    s1 = min(m)
+    s2 = max(m)
+    n = min(len(s1), len(s2))
+    for i in xrange(n):
+        if s1[i] != s2[i]:
+            return s1[:i]
+    return s1[:n]
+
 
 def expandvars(path):
     """Dummy to retain interface-compatibility with other operating systems."""