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