According to Python documentation [0], `bufsize=1` is only meaningful in text mode. As we don't pass in an argument called `universal_newlines`, `encoding`, `errors` or `text` the file objects aren't opened in text mode, which means the argument is ignored with a warning in Python 3.8. line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used This commit removes this warning that appared when using interactive test driver built with `-A driver`. This is done by removing `bufsize=1` from Popen calls. The default parameter when unspecified for `bufsize` is `-1` which according to the documentation will be interpreted as `io.DEFAULT_BUFFER_SIZE`. As mentioned by a warning, Python already uses default buffer size when providing `buffering=1` parameter for file objects not opened in text mode. [0]: https://docs.python.org/3/library/subprocess.html#subprocess.Popen