docs: 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