Index: Lib/urllib/parse.py
===================================================================
--- Lib/urllib/parse.py	(revision 81228)
+++ Lib/urllib/parse.py	(working copy)
@@ -307,17 +307,20 @@
     """unquote_to_bytes('abc%20def') -> b'abc def'."""
     # Note: strings are encoded as UTF-8. This is only an issue if it contains
     # unescaped non-ASCII characters, which URIs should not.
+    if not string:
+        return b''
     if isinstance(string, str):
         string = string.encode('utf-8')
     res = string.split(b'%')
-    res[0] = res[0]
-    for i in range(1, len(res)):
-        item = res[i]
+    if len(res) == 1:
+        return string
+    string = res[0]
+    for item in res[1:]:
         try:
-            res[i] = bytes([int(item[:2], 16)]) + item[2:]
+            string += bytes([int(item[:2], 16)]) + item[2:]
         except ValueError:
-            res[i] = b'%' + item
-    return b''.join(res)
+            string += b'%' + item
+    return string
 
 def unquote(string, encoding='utf-8', errors='replace'):
     """Replace %xx escapes by their single-character equivalent. The optional
@@ -439,6 +442,7 @@
                          b'abcdefghijklmnopqrstuvwxyz'
                          b'0123456789'
                          b'_.-')
+_ALWAYS_SAFE_BYTES = bytes(_ALWAYS_SAFE)
 _safe_quoters= {}
 
 class Quoter(collections.defaultdict):
@@ -451,7 +455,7 @@
     # of cached keys don't call Python code at all).
     def __init__(self, safe):
         """safe: bytes object."""
-        self.safe = _ALWAYS_SAFE.union(c for c in safe if c < 128)
+        self.safe = _ALWAYS_SAFE.union(safe)
 
     def __repr__(self):
         # Without this, will just display as a defaultdict
@@ -493,6 +497,8 @@
     errors='strict' (unsupported characters raise a UnicodeEncodeError).
     """
     if isinstance(string, str):
+        if not string:
+            return string
         if encoding is None:
             encoding = 'utf-8'
         if errors is None:
@@ -527,18 +533,23 @@
     not perform string-to-bytes encoding.  It always returns an ASCII string.
     quote_from_bytes(b'abc def\xab') -> 'abc%20def%AB'
     """
+    if not isinstance(bs, (bytes, bytearray)):
+        raise TypeError("quote_from_bytes() expected bytes")
+    if not bs:
+        return ''
     if isinstance(safe, str):
         # Normalize 'safe' by converting to bytes and removing non-ASCII chars
         safe = safe.encode('ascii', 'ignore')
-    cachekey = bytes(safe)  # In case it was a bytearray
-    if not (isinstance(bs, bytes) or isinstance(bs, bytearray)):
-        raise TypeError("quote_from_bytes() expected a bytes")
+    else:
+        safe = bytes([c for c in safe if c < 128])
+    if not bs.rstrip(_ALWAYS_SAFE_BYTES + safe):
+        return bs.decode()
     try:
-        quoter = _safe_quoters[cachekey]
+        quoter = _safe_quoters[safe]
     except KeyError:
-        quoter = Quoter(safe)
-        _safe_quoters[cachekey] = quoter
-    return ''.join([quoter[char] for char in bs])
+        quoter = Quoter(safe).__getitem__
+        _safe_quoters[safe] = quoter
+    return ''.join([quoter(char) for char in bs])
 
 def urlencode(query, doseq=False):
     """Encode a sequence of two-element tuples or dictionary into a URL query string.