Index: Lib/io.py
===================================================================
--- Lib/io.py	(revision 67439)
+++ Lib/io.py	(working copy)
@@ -1045,7 +1045,18 @@
                     # XXX Why not just let the exception pass through?
                     raise BlockingIOError(e.errno, e.strerror, 0)
             before = len(self._write_buf)
-            self._write_buf.extend(b)
+            bytes_to_consume = self.max_buffer_size - before
+            # b is an iterable of ints, it won't always support len().
+            if hasattr(b, '__len__') and len(b) > bytes_to_consume:
+                # Avoid an extra data copy if possible.
+                try:
+                    data = memoryview(b)[:bytes_to_consume]
+                except TypeError:
+                    # No buffer API?  slice into a temporary copy.
+                    data = b[:bytes_to_consume]
+                self._write_buf.extend(data)
+            else:
+                self._write_buf.extend(b)
             written = len(self._write_buf) - before
             if len(self._write_buf) > self.buffer_size:
                 try:
Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(revision 67439)
+++ Lib/test/test_io.py	(working copy)
@@ -425,7 +425,9 @@
             random.shuffle(l)
             s = bytes(bytearray(l))
             with io.open(support.TESTFN, "wb") as f:
-                f.write(s)
+                idx = 0
+                while idx < len(s):
+                    idx += f.write(memoryview(s)[idx:])
             with io.open(support.TESTFN, "rb", buffering=0) as raw:
                 bufio = io.BufferedReader(raw, 8)
                 errors = []