On Wed, Oct 31, 2007 at 03:55:49PM +0100, jelle feringa wrote regarding Re:
shouldn't 'string'.find('ugh') return 0, not -1 ?:
>
> There is a subtle point though.
>
> If the substring is not found '_'.find(' '), will return -1
>
> Semanticly, I was expecting the that if the substring was not found,
> the conditional statement would not be found.
>
> However, python evaluates -1 to True, so that is what I do find
> confusing.
>
> So, I was arguing that '_'.find(' ') might return 0, however that is
> obviously ambigious, since 0 might be an index as well.
>
>
>
> So, perhaps I should rephrase and ask, why if -1 evaluates to True?
>
> I think that's pretty ugly...
>
>
It is ugly, but there's no alternative. In essence, you can't do a truth-test
on it, because you're not testing the truth of the find statement. The
statement that you want to test the truth of is s.find(q) >= 0. In other
words, you want to see that the substring was found at a valid (non-negative)
location. As someone else pointed out, it would make more sense to use None
instead of -1. You still couldn't use `if s.find(q):` because you'd get a
false result for a substring found at the beginning of the string, but the
return value would make more intuitive sense.
Witness also the problem of actually using the result of find:
s[s.find(q)]
will yield a valid part of the string (the last character) when q is not found.
If it evaluated to none, you'd get a respectable TypeError instead.
But you still wouldn't--and never will--be able to say `if s.find(q)`, because
valid array indices can be either true or false.
Cheers,
Cliff
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