Index: Lib/io.py
===================================================================
--- Lib/io.py	(revision 56394)
+++ Lib/io.py	(working copy)
@@ -1056,15 +1056,19 @@
     def isatty(self):
         return self.buffer.isatty()
 
-    def write(self, s: str):
+
+    def write(self, s: (str, bytes)):
         if self.closed:
             raise ValueError("write to closed file")
         # XXX What if we were just reading?
-        b = s.encode(self._encoding)
-        if isinstance(b, str):
+        if isinstance(s, basestring):
+            b = s.encode(self._encoding)
+        elif isinstance(s, bytes):
+            b = s
+        else:
             b = bytes(b)
         n = self.buffer.write(b)
-        if "\n" in s:
+        if b"\n" in b:
             # XXX only if isatty
             self.flush()
         self._snapshot = self._decoder = None
Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(revision 56394)
+++ Lib/test/test_io.py	(working copy)
@@ -660,6 +660,20 @@
                 print("Reading using readline(): %6.3f seconds" % (t3-t2))
                 print("Using readline()+tell():  %6.3f seconds" % (t4-t3))
 
+    def testWriteBytes(self):
+        encs = ["utf-8", "latin-1"]
+        data_u = '\xc3\xbcber'
+        for enc in encs:
+            data_b = data_u.encode(enc)
+            self.failUnless(isinstance(data_b, bytes))
+            f = io.open(test_support.TESTFN, "w", encoding=enc)
+            self.failUnless(isinstance(f, io.TextIOWrapper))
+            f.write(data_b)
+            f.close()
+            f = io.open(test_support.TESTFN, "r", encoding=enc)
+            self.failUnless(isinstance(f, io.TextIOWrapper))
+            self.assertEqual(f.read(), data_u)
+            f.close()
 
 # XXX Tests for open()