Index: urllib.py
===================================================================
--- urllib.py	(revision 62222)
+++ urllib.py	(working copy)
@@ -27,6 +27,7 @@
 import os
 import time
 import sys
+import re
 from urlparse import urljoin as basejoin
 
 __all__ = ["urlopen", "URLopener", "FancyURLopener", "urlretrieve",
@@ -1175,6 +1176,7 @@
                'abcdefghijklmnopqrstuvwxyz'
                '0123456789' '_.-')
 _safemaps = {}
+_must_quote = {}
 
 def quote(s, safe = '/'):
     """quote('abc def') -> 'abc%20def'
@@ -1200,8 +1202,11 @@
     cachekey = (safe, always_safe)
     try:
         safe_map = _safemaps[cachekey]
+        if not _must_quote[cachekey].search(s):
+            return s
     except KeyError:
         safe += always_safe
+        _must_quote[cachekey] = re.compile(r'[^%s]' % safe)
         safe_map = {}
         for i in range(256):
             c = chr(i)