import string

class MyForm(string.Formatter):
    def parse(self, format_string):
        for tpl in string.Formatter.parse(self, format_string):
            print(tpl)
            literal_text, field_name, format_spec, conversion = tpl
            if field_name:
                literal_text += f'"{field_name}"'
                yield literal_text, None, None, None
            else:
                yield tpl

fmt = '{abc} {def} {3} {4} foo bar not in brackets {hello} {5,7} end'
print(fmt)
x = MyForm()
print(x.format(fmt))

"""

{abc} {def} {3} {4} foo bar not in brackets {hello} {5,7} end
('', 'abc', '', None)
(' ', 'def', '', None)
(' ', '3', '', None)
(' ', '4', '', None)
(' foo bar not in brackets ', 'hello', '', None)
(' ', '5,7', '', None)
(' end', None, None, None)
"abc" "def" "3" "4" foo bar not in brackets "hello" "5,7" end

"""