import string

class MyFormatter(string.Formatter):
    def parse(self, format_string):
        idx = 0
        auto_idx = True
        while True:
            # see if there's a {
            txt, sep, remain = format_string.partition('{')

            # todo: see if { or } are escaped

            if not sep:
                # no more substitution strings
                yield txt, None, None, None
                return
            # look for the terminationg }
            field, sep, format_string = remain.partition('}')
            if not sep:
                raise ValueError('missing matching "}"')

            # if field is empty, or the first character is
            #  a colon, the field name is missing
            if len(field) == 0 or field[0] == ':':
                field_missing = True
            else:
                field_missing = False

            if idx == 0:
                # first time through, remember which style we're using
                if field_missing:
                    auto_idx = True
                else:
                    auto_idx = False
            else:
                # make sure we use the same style every time
                if field_missing != auto_idx:
                    raise ValueError('cannot mix and match auto indexing')

            if auto_idx:
                field = str(idx) + field
            field_name, _, format_spec = field.partition(':')

            # todo: see if format_spec contains a conversion specifier
            yield txt, field_name, format_spec, None

            idx += 1

if __name__ == '__main__':
    import unittest

    import datetime

    class TestCase(unittest.TestCase):
        def setUp(self):
            self.f = MyFormatter()

        def testInvalidStrings(self):
            self.assertRaises(ValueError, self.f.format, '{')

        def testMixAndMatch(self):
            self.assertRaises(ValueError, self.f.format, '{}{0}', 1, 2)
            self.assertRaises(ValueError, self.f.format, '{0}{}', 1, 2)

        def testNormal(self):
            self.assertEqual(self.f.format('abc'), 'abc')
            self.assertEqual(self.f.format('{0}', 3), '3')
            self.assertEqual(self.f.format('{foo}', foo=3), '3')

        def testAutoIndex(self):
            self.assertEqual(self.f.format('a{}b', 10), 'a10b')
            self.assertEqual(self.f.format('a{}b{}', 10, 20), 'a10b20')
            self.assertEqual(self.f.format('a{}b{}c', 10, 'bar'), 'a10bbarc')
            self.assertEqual(self.f.format('a{:.2f}b', 10), 'a10.00b')
            self.assertEqual(self.f.format('a{:.2f}b{:.3f}c', 10, 20), 'a10.00b20.000c')

            dt = datetime.datetime(2009, 2, 14)
            self.assertEqual(self.f.format('{:%Y-%m-%d}', dt), '2009-02-14')

    unittest.main()