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

Although I am four years late, I think I have the fastest solution. Recently I wrote a python package called linereader, which allows you to manipulate the pointers of file handles.

Here is the simple solution to getting a random line with this package:

from random import randint
from linereader import dopen

length = #lines in file
filename = #directory of file

file = dopen(filename)
random_line = file.getline(randint(1, length))

The first time this is done is the worst, as linereader has to compile the output file in a special format. After this is done, linereader can then access any line from the file quickly, whatever size the file is.

If your file is very small (small enough to fit into an MB), then you can replace dopen with copen, and it makes a cached entry of the file within memory. Not only is this faster, but you get the number of lines within the file as it is loaded into memory; it is done for you. All you need to do is to generate the random line number. Here is some example code for this.

from random import randint
from linereader import copen

file = copen(filename)
lines = file.count('\n')
random_line = file.getline(randint(1, lines))

I just got really happy because I saw someone who could benefit from my package! Sorry for the dead answer, but the package could definitely be applied to many other problems.