Watch a text file for new content (a-la "tail -f"); reload file if it seems to have been truncated

filewatch.py

from time import sleep


def watch_fp(fp, delim: str = "\n"):
    buf = []
    while True:
        raw_input = fp.read()
        if raw_input != "":
            buf.append(raw_input)
            if delim in raw_input:
                units = ''.join(buf).split(delim)
                buf = [units.pop()]  # unterminated part
                yield from units
        else:
            yield None  # no new data


def watch(filename: str):
    while True:
        with open(filename, 'r') as fp:
            fetch = watch_fp(fp)
            is_file_reset = False
            while not is_file_reset:
                # fetch and print new data (if any)
                next_line = next(fetch)
                while next_line is not None:
                    print(next_line)
                    next_line = next(fetch)

                # check if file was reset
                position = fp.tell()
                fp.seek(0, 2)
                new_position = fp.tell()
                is_file_reset = new_position < position

                # nothing new
                if not is_file_reset:
                    sleep(1)

        print("--- RELOADING FILE ---")


def main():
    args = parse_args()
    watch(args.filename)


def parse_args():
    import argparse
    parser = argparse.ArgumentParser('filewatch', description='file change tracker')
    parser.add_argument('filename', type=str, help="file to watch")
    args = parser.parse_args()
    return args


if __name__ == '__main__':
    main()