file_indexing.py

from pathlib import Path
from typing import List


class IndexedFile:
    def __init__(self, path: Path) -> None:
        self.path = path
        self.offset: List[int] = []
        self._build_index()
    
    def _build_index(self) -> None:
        cumlen = 0
        with open(self.path) as fp:
            for line in fp:
                self.offset.append(cumlen)
                cumlen += len(line)

    def get_line(self, k: int) -> str:
        assert k < len(self.offset)
        with open(self.path) as fp:
            fp.seek(self.offset[k])
            line = fp.readline()
        return line


if __name__ == "__main__":
    idxfile = IndexedFile(Path("./temp.txt"))
    print(idxfile.get_line(3))
    print(idxfile.get_line(2))
    print(idxfile.get_line(1))
    print(idxfile.get_line(0))