def conversion(self, idx):
        """
        Switches into the 'conversion' state, used to parse a %
        conversion.

        :param idx: The format string index at which the conversion
                    begins.
        """

        self.state.append('conversion')
        self.str_begin = idx
        self.param_begin = None
        self.conv_begin = None
        self.modifier = conversions.Modifier()
        self.codes = []
        self.reject = False
        self.code_last = False
Switches into the 'conversion' state, used to parse a %
        conversion.

        :param idx: The format string index at which the conversion
                    begins.