[Python-Dev] Dafanging the find() gotcha

[Tim Peters]

[Aahz]
> It's not so much that '' in 'abc' is a mistake as that there's no
> sensible answer to be given.

    s2.find(s1)

returns the smallest non-negative int i such that

    s2[i : i+len(s1] == s1

provided such an i exists.  That's as sensible as any answer, and more
sensible than most <wink>, if you have to give a meaning when s1 is empty.

> When Python can't figure out how to deliver a sensible answer,

Well, i==0 isn't a *compelling* answer when s1=="".  "It falls out of the
forumla" is about the best that can be said for it.

> it raises an exception: "In the face of ambiguity, refuse the temptation
> to guess."

That's pretty much my view.  The user has just given us reason to doubt they
know what their program is doing, and I'd rather be *helpful* then than push
on in the interest of purity.

The most plausible use case I've been able to dream up is representing small
finite sets as sorted strings of characters.  Then having

    s1 in s2

raise an exception when s1 is "" doesn't do the right thing for "the empty
set".  OTOH, it doesn't do the right thing in most other cases either, like

    "ac" in "abc" -> False

so it's hard to get too upset about the empty set failing <wink>.