How do I read a random line from one file?

A slightly improved version of the Alex Martelli's answer, which handles empty files (by returning a default value):

from random import randrange

def random_line(afile, default=None):
    line = default
    for i, aline in enumerate(afile, start=1):
        if randrange(i) == 0:  # random int [0..i)
            line = aline
    return line

This approach can be used to get a random item from any iterator using O(n) time and O(1) space.

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.

import random

with open("file.txt", "r") as f:
    lines = f.readlines()
    print (random.choice(lines))