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So, what should '%s' % {} do?  Should it be 1) '{}' or 2) an
error because the argument is a mapping but the format
specifier doesn't have a '(key)'?

I've attached a draft patch that fixes stringobject.c; if
the approach is deemed OK, I'll apply it to unicodeobject.c,
too.  PyString_Format() records the type of argument being
processed (a tuple or a mapping) and raises ValueError if
you mix them, at the cost of two extra comparisons for each
format specifier processed.  This preserves the current
behaviour of '%s' % dictionary.

Questions: 1) is the approach reasonably clear?  2) are the
additional two comparisons unacceptably slow?  3) Is
ValueError the right exception?  4) can someone come up with
a better exception message than "both keyed and unkeyed
format specifiers used"?