[DylanDmitri]

In loopless Python:

    set(open('file_b')) - set(open('file_a'))

Slower than J by a factor of 2-3, but still 10x faster than grep:

    real    0m0.128s
    user    0m0.078s
    sys     0m0.063s

This would make a good Rosetta Code prompt.

[throwaway_pdp09]

I didn't know you could simply open a file and setify it. Interesting. & neat.

[fennecfoxen]

You can setify any iterable. File handles are iterables that return a line at a time. Tada!

[throwaway_pdp09]

> File handles are iterables...

I did not know that. Assumed you had to somehow wrap them first. Very useful, thanks!

[qmmmur]

this is cheeky, I like it

[dunefox]

This is only fast because it hits C underneath, isn't it?