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TextIOWrapper(b, newline="\n", line_buffering=True) object calls flush() 
while writing "\r". See test_line_buffering() method in 
Lib/test/test_io.py:2114
The documentation says [1]:
> If line_buffering is True, flush() is implied when a call to write contains
> a newline character.
i.e., writing \r shouldn't force flush() if newline="\n"
[1] https://docs.python.org/3.4/library/io.html#io.TextIOWrapper