An implementation of the 'tail -f' command in Python also supporting "file truncation" (similar to the original 'tail -f' command).

tail.py

import os
import sys
import time

from typing import Iterator, IO


def tail_follow(file_path: str, polling_interval_seconds: float = 0.1) -> Iterator[str]:
    """
    Simplified 'tail -f' implementation in Python.
    Based on https://stackoverflow.com/a/54263201
    Additions:
    - handling of file permission errors
    - handling of file truncation
    """

    def _get_file_size(file_object: IO) -> int:
        return os.fstat(file_object.fileno()).st_size

    if not os.path.isfile(file_path):
        print(f"File not found: '{file_path}'", file=sys.stderr)
        sys.exit(1)

    if not os.access(file_path, os.R_OK):
        print(f"File not accessible: '{file_path}'", file=sys.stderr)
        sys.exit(1)

    try:
        with open(file_path, "r") as file_to_follow:
            line_buffer = ""
            latest_size = _get_file_size(file_to_follow)

            while True:
                new_size = _get_file_size(file_to_follow)
                if new_size < latest_size:
                    # As in coreutils 'tail' we seek back to zero "just in case", see:
                    # https://github.com/coreutils/coreutils/blob/master/src/tail.c#L1272
                    print(
                        f"File truncated: '{file_to_follow.name}', resetting file pointer to file beginning.",
                        file=sys.stderr,
                    )
                    file_to_follow.seek(0)

                latest_size = new_size
                latest_read_line = file_to_follow.readline()

                if latest_read_line is not None and latest_read_line != "":
                    line_buffer += latest_read_line
                    if line_buffer.endswith("\n"):
                        yield line_buffer
                        line_buffer = ""

                elif polling_interval_seconds:
                    time.sleep(polling_interval_seconds)

    except KeyboardInterrupt:
        print("Interrupted by user")

    except Exception as e:
        print(f"An error occurred: {e}")
        sys.exit(1)


if __name__ == "__main__":
    for line in tail_follow(file_path=sys.argv[1]):
        print(line, end="")