tail

[Marco Sulla]

I have a little problem.

I tried to extend the tail function, so it can read lines from the bottom
of a file object opened in text mode.

The problem is it does not work. It gets a starting position that is lower
than the expected by 3 characters. So the first line is read only for 2
chars, and the last line is missing.

import os

_lf = "\n"
_cr = "\r"
_lf_ord = ord(_lf)

def tail(f, n=10, chunk_size=100):
    n_chunk_size = n * chunk_size
    pos = os.stat(f.fileno()).st_size
    chunk_line_pos = -1
    lines_not_found = n
    binary_mode = "b" in f.mode
    lf = _lf_ord if binary_mode else _lf

    while pos != 0:
        pos -= n_chunk_size

        if pos < 0:
            pos = 0

        f.seek(pos)
        chars = f.read(n_chunk_size)

        for i, char in enumerate(reversed(chars)):
            if char == lf:
                lines_not_found -= 1

                if lines_not_found == 0:
                    chunk_line_pos = len(chars) - i - 1
                    print(chunk_line_pos, i)
                    break

        if lines_not_found == 0:
            break

    line_pos = pos + chunk_line_pos + 1

    f.seek(line_pos)

    res = b"" if binary_mode else ""

    for i in range(n):
        res += f.readline()

    return res

Maybe the problem is 1 char != 1 byte?