How do I read a random line from one file?

Question

Is there a built-in method to do it? If not how can I do this without costing too much overhead?

Answer 1

If you don't want to read over the entire file, you can seek into the middle of the file, then seek backwards for the newline, and call readline.

Here is a Python3 script which does just this,

One disadvantage with this method is short lines have lower likelyhood of showing up.

def read_random_line(f, chunk_size=16):
    import os
    import random
    with open(f, 'rb') as f_handle:
        f_handle.seek(0, os.SEEK_END)
        size = f_handle.tell()
        i = random.randint(0, size)
        while True:
            i -= chunk_size
            if i < 0:
                chunk_size += i
                i = 0
            f_handle.seek(i, os.SEEK_SET)
            chunk = f_handle.read(chunk_size)
            i_newline = chunk.rfind(b'\n')
            if i_newline != -1:
                i += i_newline + 1
                break
            if i == 0:
                break
        f_handle.seek(i, os.SEEK_SET)
        return f_handle.readline()