Why doesn't join() call str() on its arguments?

[John Roth]

"Leo Breebaart" <leo at lspace.org> wrote in message 
news:37hippF4gcimiU1 at individual.net...
> I've tried Googling for this, but practically all discussions on
> str.join() focus on the yuck-ugly-shouldn't-it-be-a-list-method?
> issue, which is not my problem/question at all.
>
> What I can't find an explanation for is why str.join() doesn't
> automatically call str() on its arguments, so that e.g.
> str.join([1,2,4,5]) would yield "1245", and ditto for e.g.
> user-defined classes that have a __str__() defined.
>
> All I've been able to find is a 1999 python-dev post by Tim
> Peters which would seem to indicate he doesn't understand it
> either:
>
>    "string.join(seq) doesn't currently convert seq elements to
>     string type, and in my vision it would. At least three of us
>     admit to mapping str across seq anyway before calling
>     string.join, and I think it would be a nice convenience
>     [...]"
>
> But now it's 2005, and both string.join() and str.join() still
> explicitly expect a sequence of strings rather than a sequence of
> stringifiable objects.
>
> I'm not complaining as such -- sep.join(str(i) for i in seq) is
> not *that* ugly, but what annoys me is that I don't understand
> *why* this was never changed. Presumably there is some
> counter-argument involved, some reason why people preferred the
> existing semantics after all. But for the life of me I can't
> think what that counter-argument might be...
>
> -- 
> Leo Breebaart  <leo at lspace.org>