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The semantics the docs suggest for index fields (namely that whatever is in the index field is just passed to getitem) do seem to be right, no other processing is done here, for instance:
>>> d = {"{0}":"hi"}
>>> "{0[{0}]}".format(d)
'hi'
>>> import string
>>> list(string.Formatter().parse("{0[{0}]}"))
[('', '0[{0}]', '', None)]
>>> 
Which is what you'd expect, but makes me think that treating "!" and ":" in the index field separately is definitely wrong.