diff -r d083b6ead156 Lib/urllib/parse.py
--- a/Lib/urllib/parse.py	Mon Mar 10 11:05:07 2014 +0100
+++ b/Lib/urllib/parse.py	Mon Mar 10 13:40:17 2014 +0200
@@ -472,8 +472,7 @@
     return _coerce_result(DefragResult(defrag, frag))
 
 _hexdig = '0123456789ABCDEFabcdef'
-_hextobyte = {(a + b).encode(): bytes([int(a + b, 16)])
-              for a in _hexdig for b in _hexdig}
+_hextobyte = None
 
 def unquote_to_bytes(string):
     """unquote_to_bytes('abc%20def') -> b'abc def'."""
@@ -490,6 +489,10 @@
         return string
     res = [bits[0]]
     append = res.append
+    global _hextobyte
+    if _hextobyte is None:
+        _hextobyte = {(a + b).encode(): bytes([int(a + b, 16)])
+                      for a in _hexdig for b in _hexdig}
     for item in bits[1:]:
         try:
             append(_hextobyte[item[:2]])