It may also be worth explicitly stating the following in the docs: "Note
that automatically numbered and explcitly namged/numbered replacement
fields cannot be mixed in a single format string". (This could probably
be relegated to a footnote).

This proposal is also significantly better defined than the rather
bizarre behaviour seen in the equivalent situation with %-formatting:

>>> "%s %(name)s" % dict(name="Hmm")
"{'name': 'Hmm'} Hmm"

>>> "%s %(name)s %s" % dict(name="Hmm")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: not enough arguments for format string

>>> "%s %(name)s %s" % (dict(name="Hmm"), "dodgy")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: format requires a mapping