diff -r de2a0cb6ba52 Lib/base64.py
--- a/Lib/base64.py	Fri Jun 22 18:32:07 2012 +0200
+++ b/Lib/base64.py	Fri Jun 22 09:51:31 2012 -0700
@@ -40,12 +40,18 @@
     else:
         raise TypeError("argument should be bytes or ASCII string, not %s" % s.__class__.__name__)
 
+
+def _translation_table(altchars):
+    translation = bytearray(range(256))
+    for k, v in altchars.items():
+        translation[ord(k)] = v[0]
+    return translation
+
+
 def _translate(s, altchars):
     if not isinstance(s, bytes_types):
         raise TypeError("expected bytes, not %s" % s.__class__.__name__)
-    translation = bytearray(range(256))
-    for k, v in altchars.items():
-        translation[ord(k)] = v[0]
+    translation = _translation_table(altchars)
     return s.translate(translation)
 
 
@@ -116,6 +122,10 @@
     """
     return b64decode(s)
 
+
+_urlsafe_encode_translation = _translation_table({'+': b'-', '/': b'_'})
+_urlsafe_decode_translation = _translation_table({'-': b'+', '_': b'/'})
+
 def urlsafe_b64encode(s):
     """Encode a byte string using a url-safe Base64 alphabet.
 
@@ -123,7 +133,7 @@
     returned.  The alphabet uses '-' instead of '+' and '_' instead of
     '/'.
     """
-    return b64encode(s, b'-_')
+    return b64encode(s).translate(_urlsafe_encode_translation)
 
 def urlsafe_b64decode(s):
     """Decode a byte string encoded with the standard Base64 alphabet.
@@ -135,8 +145,9 @@
 
     The alphabet uses '-' instead of '+' and '_' instead of '/'.
     """
-    return b64decode(s, b'-_')
-
+    s = _bytes_from_decode_data(s)
+    s = s.translate(_urlsafe_decode_translation)
+    return b64decode(s)
 
 
 # Base32 encoding/decoding must be done in Python