Get last n lines of a file, similar to tail

Question

I\'m writing a log file viewer for a web application and for that I want to paginate through the lines of the log file. The items in the file are line based with the newest

Answer 1

An even cleaner python3 compatible version that doesn't insert but appends & reverses:

def tail(f, window=1):
    """
    Returns the last `window` lines of file `f` as a list of bytes.
    """
    if window == 0:
        return b''
    BUFSIZE = 1024
    f.seek(0, 2)
    end = f.tell()
    nlines = window + 1
    data = []
    while nlines > 0 and end > 0:
        i = max(0, end - BUFSIZE)
        nread = min(end, BUFSIZE)

        f.seek(i)
        chunk = f.read(nread)
        data.append(chunk)
        nlines -= chunk.count(b'\n')
        end -= nread
    return b'\n'.join(b''.join(reversed(data)).splitlines()[-window:])

use it like this:

with open(path, 'rb') as f:
    last_lines = tail(f, 3).decode('utf-8')