"%s" vs unicode

[Robin Becker]

A user reports that an old style error message is being smashed as the
message was being turned into unicode.

the situation was of the form

raise "xxxx %s' % var

where var was a lump of unicode. We can easily correct the sistuation.

We didn't expect unicode, but what's surprising is that "%s" % u'A' -->
u'A'.

Doesn't %s mean take the str()?

I looked in vain in the docs for the definition of exactly what "%s" % x
is supposed to mean, but somehow this kind of reminds me of the old /
operator dispute ie int / int --> int & int / float --> float.

That was considered harmful (although I disagreed) so why is this
implicit conversion allowed? 
-- 
Robin Becker