This is the result that I see:

>>> output = StringIO()
>>> csv.writer(output, lineterminator='\n').writerow(["Whoa!\rNewlines!"])
16
>>> output.getvalue()
'Whoa!\rNewlines!\n'

For comparison, this is the result with CRLF terminators (the default):

>>> output = StringIO()
>>> csv.writer(output, lineterminator='\r\n').writerow(["Whoa!\rNewlines!"])
19
>>> output.getvalue()
'"Whoa!\rNewlines!"\r\n'

Is it a problem that the line terminator determines whether the CR is quoted or not? I believe the default policy is “excel”, which happens to use QUOTE_MINIMAL. This behaviour is documented: <https://docs.python.org/3.7/library/csv.html#csv.QUOTE_MINIMAL>.