import random
filesize = 1500 #size of the really big file
offset = random.randrange(filesize)
f = open('really_big_file')
f.seek(offset) #go to random position
f.readline() # discard - bound to be partial line
random_line = f.readline() # bingo!
# extra to handle last/first line edge cases
if len(random_line) == 0: # we have hit the end
f.seek(0)
random_line = f.readline() # so we'll grab the first line instead
As @AndreBoos pointed out, this approach will lead to biased selection. If you know min and max length of line you can remove this bias by doing the following:
Let’s assume (in this case) we have min=3 and max=15
1) Find the length (Lp) of the previous line.
Then if Lp = 3, the line is most biased against. Hence we should take it 100% of the time
If Lp = 15, the line is most biased towards. We should only take it 20% of the time as it is 5* more likely selected.
We accomplish this by randomly keeping the line X% of the time where:
X = min / Lp
If we don’t keep the line, we do another random pick until our dice roll comes good. 🙂