import random

def randomLines(filename, lines=1):
    selected_lines = list(None for line_no in xrange(lines))

    for line_index, line in enumerate(open(filename)):
        for selected_line_index in xrange(lines):
            if random.uniform(0, line_index) < 1:
                selected_lines[selected_line_index] = line

    return selected_lines

This has the advantage that every line had the same chance of being
picked regardless of its length. There is the chance that it'll pick
the same line more than once, though.