related: msg217596 (bufsize=1 is broken if subprocess module uses Python io)

TextIOWrapper.write behavior:

_pyio.py 1:

        if self._line_buffering and (haslf or "\r" in s):
            self.flush()

textio.c 2:

if (self->write_through)
    needflush = 1;
else if (self->line_buffering &&
    (haslf ||
     PyUnicode_FindChar(text, '\r', 0, PyUnicode_GET_LENGTH(text), 1) != -1))
    needflush = 1;

C implementation calls flush() if write_through=True, Python implementation doesn't.

Thanks for the report, Akira. Feel free to submit a patch! (with a test)

I've uploaded the patch that makes C implementation behave according
to the docs like Python implementation with the corresponding tests.

Issue bpo-21332 is a dependency for this issue: subprocess' test_universal_newlines needs to be updated to work with Python version.

For Reference
-------------

issue bpo-12591 introduces write_through to support subprocess' stdin
pipe in text mode with bufsize=0 i.e., TextIOWrapper.buffer is
unbuffered (raw) and Python and C implementation behave the same in
this particular case.

C implementation (pseudo-code)::

  if self.write_through:
      flush_text_buffer()
      buffer.flush() # <-- undocumented

Python implementation::

   if self.write_through:
       pass # _pyio.TextIOWrapper.write() calls buffer.write() directly

behaves according to the current documentation 1:

If *write_through* is ``True``, calls to :meth:`write` are guaranteed
not to be buffered: any data written on the :class:`TextIOWrapper`
object is immediately handled to its underlying binary *buffer*

For reference, here's how subprocess.py uses write_through 2::

  self.stdin = io.open(pipe, 'wb', bufsize)
  if universal_newlines=True:
      self.stdin = io.TextIOWrapper(self.stdin,
          write_through=True,
          line_buffering=(bufsize == 1)) # <-- issue python/cpython#65531

http://hg.python.org/cpython/rev/9ce8fa0a0899/ - introduce io.TextIOWrapper in subprocess
http://hg.python.org/cpython/rev/5cc536fbd7c1 - introduce write_through

The patch looks basically fine. I will make a few tweaks to the comments in the test case.

Actually, with the patch, the universal_newlines tests in test_subprocess hang (quite logically, since they lack a flush()). I will fix them as well.

New changeset 39f2a78f4357 by Antoine Pitrou in branch '3.4':
Issue bpo-21396: Fix TextIOWrapper(..., write_through=True) to not force a flush() on the underlying binary stream.
http://hg.python.org/cpython/rev/39f2a78f4357

New changeset 37d0c41ed8ad by Antoine Pitrou in branch 'default':
Issue bpo-21396: Fix TextIOWrapper(..., write_through=True) to not force a flush() on the underlying binary stream.
http://hg.python.org/cpython/rev/37d0c41ed8ad

Thank you for the patch!