tail py

#!/usr/bin/python2


"""Tests different versions to implement tail (except seek or mmap)

Results for python-2.7, linux x86_64:

20940 lines, average line length 163:
dequetail       3.69968
listtail        5.02768

Performance seems largely independent of requested number of lines
"""

import collections
import sys
import timeit


def tail(filename, count, method):
    with open(filename, 'r') as filein:
        return method(filein, count)


def dequetail(filein, count):
    return collections.deque(filein, count)


def listtail(filein, count):
    lines = list()
    readlines = filein.readlines
    while True:
        block = readlines(8196)
        if block:
            replaced = max(0, len(block) + len(lines) - count)
            del lines[:replaced]
            lines += block
        else:
            break
    del lines[:-count]
    return lines


def test(methods, filename, count):
    results = ((method, list(tail(filename, count, method))) for method
               in methods)
    _, trusted = next(results)
    wrongs = (method.__name__ for method, result in results
              if result != trusted)
    messages = ("%s failed\n" % method for method in wrongs)
    sys.stderr.writelines(messages)


filename = None

def main():
    global filename
    filename = sys.argv[1]
    count = 100
    methods = (dequetail, listtail)
    test(methods, filename, count)
    methodnames = [method.__name__ for method in methods]
    setups = ('from __main__ import %s, tail, filename' % method for method
              in methodnames)
    stmts = ("tail(filename, %d, %s)" % (count, method) for method
             in methodnames)
    times = (timeit.timeit(setup=setup, stmt=stmt, number=1000)
             for setup, stmt in zip(setups, stmts))
    output = ("%s\t%g\n" % (method, time) for method, time
              in zip(methodnames, times))
    sys.stderr.writelines(output)


if __name__ == '__main__':
    main()