find the last n lines of a text file assumed to end with a newline character

last_n_lines.py

from pathlib import Path

def reverse_readline(filepath, N):
    with filepath.open('rb') as f:
        f.seek(0, 2)
        filesize = f.tell()
        lines_found = []
        buffer = bytearray()
        chunk_size = 8192  # Read in chunks of 8KB
        position = filesize

        while position >= 0 and len(lines_found) < N:
            position -= chunk_size
            if position < 0:
                chunk_size += position  # Adjust chunk_size if it overshoots the start of the file
                position = 0
            f.seek(position)
            chunk = f.read(chunk_size) + buffer

            lines = chunk.split(b'\n')
            # Process all lines found, except potentially incomplete first line in chunk
            for i in range(len(lines) - 1, 0, -1):
                lines_found.append(lines[i].decode(errors='ignore'))
                if len(lines_found) == N:
                    break
            buffer = lines[0]  # Handle any incomplete line by prepending it to the buffer for the next chunk

        if buffer:
            # Ensure any remaining buffer (e.g., the first line of the file) is included
            lines_found.append(buffer.decode(errors='ignore'))

    return reversed(lines_found)

filepath = Path('file.txt')
last_n_lines = reverse_readline(filepath, N=5)

for line in last_n_lines:
    print(line)

In [3]: cat file.txt
foo\n
foo\n
foo\n
foo\n
foo\n
foo\n
foo\n
bar\n
baz\n

In [4]: last_n_lines = reverse_readline(filepath, N=5)

In [5]: for line in last_n_lines:
   ...:     print(line)
   ...:
foo\n
foo\n
foo\n
bar\n
baz\n