https://docs.python.org/3.3/library/subprocess.html says

"bufsize will be supplied as the corresponding argument to the open() function when creating the stdin/stdout/stderr pipe file objects: ... 1 means line buffered"

but it calls io.open with 'wb' and 'rb': http://hg.python.org/cpython/file/c9cb931b20f4/Lib/subprocess.py#l828

This puts the file in binary mode, and https://docs.python.org/3.3/library/functions.html#open says

"1 to select line buffering (only usable in text mode)"

Even with universal_newlines=True, the TextIOWrapper isn't passed line_buffering=True. There's actually no way to get line buffering any more.