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On 3.2 subprocess.Popen.communicate with universal_newlines=True accepts bytes and doesn't accept strings. 
$ ./python -c "import subprocess; subprocess.Popen(['cat'], stdin=subprocess.PIPE, stdout=subprocess.PIPE, universal_newlines=True).communicate('qwerty')"
Traceback (most recent call last):
 File "<string>", line 1, in <module>
 File "/home/serhiy/py/cpython3.2/Lib/subprocess.py", line 833, in communicate
 return self._communicate(input)
 File "/home/serhiy/py/cpython3.2/Lib/subprocess.py", line 1470, in _communicate
 stdout, stderr = self._communicate_with_poll(input)
 File "/home/serhiy/py/cpython3.2/Lib/subprocess.py", line 1537, in _communicate_with_poll
 input_offset += os.write(fd, chunk)
TypeError: 'str' does not support the buffer interface
On 3.3+ it accepts strings and doesn't accept bytes.

As quick fix I propose add support of strings (in additional to bytes) in 3.2. Perhaps we must also add support of bytes with warning in 3.3.

Here is a patch which adds support of strings in communicate(). It also contains a backported test from changeset 4af2c0687970 which tests this behavior.

I would suggest that this be bumped to release blocker as currently passing bytes to communicate with universal_newlines=True on Windows causes the process to hang (see issue17077).
The backported test case passes on Windows without any changes to subprocess.py so this is a POSIX-only bug.

Indeed, Popen.communicate() is incompatible not only between 3.2 and 3.3, but between Windows and POSIX too.
Documentation says the input argument should be a bytes string and 3.2 on POSIX works only with bytes strings. 3.2 on Windows and 3.3 on both platforms require an unicode string, as 3.3 documentation says. 3.2 on Windows hangs with a bytes string.
Patch updated with the documentation change (copied from 3.3).

New changeset 4206f91c974c by Serhiy Storchaka in branch '3.2':
Issue #16903: Popen.communicate() on Unix now accepts strings when
http://hg.python.org/cpython/rev/4206f91c974c