diff -r c7806d1b09eb Lib/base64.py
--- a/Lib/base64.py	Sun Apr 21 10:10:50 2013 -0400
+++ b/Lib/base64.py	Mon Apr 22 00:30:11 2013 +0300
@@ -166,7 +166,7 @@
     if leftover:
         s = s + bytes(5 - leftover)  # Don't use += !
         quanta += 1
-    encoded = bytes()
+    encoded = []
     for i in range(quanta):
         # c1 and c2 are 16 bits wide, c3 is 8 bits wide.  The intent of this
         # code is to process the 40 bits in units of 5 bits.  So we take the 1
@@ -176,25 +176,25 @@
         c1, c2, c3 = struct.unpack('!HHB', s[i*5:(i+1)*5])
         c2 += (c1 & 1) << 16 # 17 bits wide
         c3 += (c2 & 3) << 8  # 10 bits wide
-        encoded += bytes([_b32tab[c1 >> 11],         # bits 1 - 5
-                          _b32tab[(c1 >> 6) & 0x1f], # bits 6 - 10
-                          _b32tab[(c1 >> 1) & 0x1f], # bits 11 - 15
-                          _b32tab[c2 >> 12],         # bits 16 - 20 (1 - 5)
-                          _b32tab[(c2 >> 7) & 0x1f], # bits 21 - 25 (6 - 10)
-                          _b32tab[(c2 >> 2) & 0x1f], # bits 26 - 30 (11 - 15)
-                          _b32tab[c3 >> 5],          # bits 31 - 35 (1 - 5)
-                          _b32tab[c3 & 0x1f],        # bits 36 - 40 (1 - 5)
-                          ])
+        encoded.append(bytes([_b32tab[c1 >> 11],         # bits 1 - 5
+                              _b32tab[(c1 >> 6) & 0x1f], # bits 6 - 10
+                              _b32tab[(c1 >> 1) & 0x1f], # bits 11 - 15
+                              _b32tab[c2 >> 12],         # bits 16 - 20 (1 - 5)
+                              _b32tab[(c2 >> 7) & 0x1f], # bits 21 - 25 (6 - 10)
+                              _b32tab[(c2 >> 2) & 0x1f], # bits 26 - 30 (11 - 15)
+                              _b32tab[c3 >> 5],          # bits 31 - 35 (1 - 5)
+                              _b32tab[c3 & 0x1f],        # bits 36 - 40 (1 - 5)
+                              ]))
     # Adjust for any leftover partial quanta
     if leftover == 1:
-        return encoded[:-6] + b'======'
+        encoded[-1] = encoded[-1][:-6] + b'======'
     elif leftover == 2:
-        return encoded[:-4] + b'===='
+        encoded[-1] = encoded[-1][:-4] + b'===='
     elif leftover == 3:
-        return encoded[:-3] + b'==='
+        encoded[-1] = encoded[-1][:-3] + b'==='
     elif leftover == 4:
-        return encoded[:-1] + b'='
-    return encoded
+        encoded[-1] = encoded[-1][:-1] + b'='
+    return b''.join(encoded)
 
 
 def b32decode(s, casefold=False, map01=None):