There is a bug in the line buffering code in the
Lib/socket.py module.

The write() method in the _fileobject class will flush
the buffered data at every call even if there no new
line character present in the data.

The problem is that the third part of the last if
statement will always be True if self._wbufsize equals 1.

<pre>
  if (self._wbufsize == 0 or
      self._wbufsize == 1 and '\n' in data or
      self._get_wbuf_len() >= self._wbufsize):
      self.flush()
</pre>

A possible fix would be the following:

<pre>
  if (self._wbufsize == 0 or
      self._wbufsize == 1 and '\n' in data or
      self._wbufsize > 1 and self._get_wbuf_len() >=
self._wbufsize):
      self.flush()
</pre>

The code described by Jean is still present.

I'm just wondering if the original code could be the cause of subtle bugs with socket.py, anyone?  (I'm no sockets guru). There's a proposed inline patch that's never been implemented, what do you (plural) make of it?.

Here's a patch for the current 2.7 stuff, from issue #16680 (which was deemed duplicate).
Unless anyone objects, I'll commit that to 2.7 soon.  Eight years, this has taken.

Would be nice to add a test...

Good point, will do.

New patch, with unittest.

I just have updated patch for current version of code. It looks good for me.

LGTM.
Kristján, would you like to commit?

Sure, Leave it to me.

New changeset 5be3fa83d436 by Kristján Valur Jónsson in branch '2.7':
issue #879399
http://hg.python.org/cpython/rev/5be3fa83d436