def _extract_format_keys(self, fmt_string):
        """Extract the keys of a format string
        (the 'stuff' in '%(stuff)s'
        """

        class AccessSaver:
            def __init__(self):
                self.keys = []

            def __getitem__(self, key):
                self.keys.append(key)

        a = AccessSaver()
        fmt_string % a

        return a.keys