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

If reading the whole file is acceptable then use a deque.

from collections import deque
deque(f, maxlen=n)

Prior to 2.6, deques didn't have a maxlen option, but it's easy enough to implement.

import itertools
def maxque(items, size):
    items = iter(items)
    q = deque(itertools.islice(items, size))
    for item in items:
        del q[0]
        q.append(item)
    return q

If it's a requirement to read the file from the end, then use a gallop (a.k.a exponential) search.

def tail(f, n):
    assert n >= 0
    pos, lines = n+1, []
    while len(lines) <= n:
        try:
            f.seek(-pos, 2)
        except IOError:
            f.seek(0)
            break
        finally:
            lines = list(f)
        pos *= 2
    return lines[-n:]