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: