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Here are some related failing cases that I found:
>>> f = string.Formatter()
>>> f.format(u"{0}", "")
''
>>> f.format(u"{0}", 1)
'1'
>>> f.format(u"{0}", "a")
'a'
>>> f.format(u"{0}{1}", "a", "b")
'ab'
>>> f.format("{0}", u"a") 
u'a'
Note that PEP 3101 says the following:
"In all cases, the type of the format string dominates - that
is, the result of the conversion will always result in an object
that contains the same representation of characters as the
input format string."