Sorting in huge files

[Paul]

I really do need to sort. It is complicated and I haven't said why, but
it will help in finding similar keys later on. Sorry I can't be more
precise, this has to do with my research.

Your two other suggestions with itertools and operator are more useful,
but I was mostly wondering about performance issue.

Is this reasonnable to do on 10^8 elements with repeats in the keys? I
guess I should just try and see for myself.