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 load the whole file into RAM with f.read() or f.readlines(), you can get random line this way:

import os
import random


def get_random_line(filepath: str) -> str:
    file_size = os.path.getsize(filepath)
    with open(filepath, 'rb') as f:
        while True:
            pos = random.randint(0, file_size)
            if not pos:  # the first line is chosen
                return f.readline().decode()  # return str
            f.seek(pos)  # seek to random position
            f.readline()  # skip possibly incomplete line
            line = f.readline()  # read next (full) line
            if line:
                return line.decode()  
            # else: line is empty -> EOF -> try another position in next iteration

P.S.: yes, that was proposed by Ignacio Vazquez-Abrams in his answer above, but a) there's no code in his answer and b) I've come up with this implementation myself; it can return first or last line. Hope it may be useful for someone.

However, if you care about distribution, this code is not an option for you.