Index: Lib/io.py
===================================================================
--- Lib/io.py	(revision 67439)
+++ Lib/io.py	(working copy)
@@ -1042,11 +1042,41 @@
                     self._flush_unlocked()
                 except BlockingIOError as e:
                     # We can't accept anything else.
-                    # XXX Why not just let the exception pass through?
+                    # Reraise this with 0 in the written field as none of the
+                    # data passed to this call has been written.
                     raise BlockingIOError(e.errno, e.strerror, 0)
             before = len(self._write_buf)
-            self._write_buf.extend(b)
-            written = len(self._write_buf) - before
+            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:
+                try:
+                    chunk = memoryview(b)[:bytes_to_consume]
+                except TypeError:
+                    # No buffer API?  Make intermediate slice copies instead.
+                    chunk = b[:bytes_to_consume]
+                # Loop over the data, flushing it to the underlying raw IO
+                # stream in self.max_buffer_size chunks.
+                written = 0
+                while chunk:
+                    self._write_buf.extend(chunk)
+                    try:
+                        self._flush_unlocked()
+                    except BlockingIOError as e:
+                        written += e.characters_written
+                        raise BlockingIOError(e.errno, e.strerror, written)
+                    written += len(chunk)
+                    if isinstance(chunk, memoryview):
+                        chunk = memoryview(b)[written:
+                                              written + self.max_buffer_size]
+                    else:
+                        chunk = b[written:written + self.max_buffer_size]
+                    assert not self._write_buf, "_write_buf should be empty"
+            else:
+                # This could go beyond self.max_buffer_size as we don't know
+                # the length of b.  The alternative of iterating over it one
+                # byte at a time in python would be slow.
+                self._write_buf.extend(b)
+                written = len(self._write_buf) - before
             if len(self._write_buf) > self.buffer_size:
                 try:
                     self._flush_unlocked()