Ah, thanks, I noticed the discrepancy between unicode and str in 2.7, but wondered when it was fixed. I guess I'm arguing that it was resolved in the wrong direction, then.

Now, your first expression is wrong, even after fixing the obvious typo. The correct version is:
    start + len(s2) <= min(len(s1), end) and s1[start: start + len(s2)] == s2

If the person who implemented the behaviour can get it right, who will? ;-)

The second expression is correct, but I'll argue that it shows that find() also suffers from a discrepancy between its basic one-argument form and the extended ones.