if conversion is None:
            return value
        elif conversion == 's':
            return str(value)
        elif conversion == 'r':
            return repr(value)
        elif conversion == 'a':
            return ascii(value)
        raise ValueError("Unknown conversion specifier {0!s}".format(conversion))
    # returns an iterable that contains tuples of the form:
    # (literal_text, field_name, format_spec, conversion)
    # literal_text can be zero length
    # field_name can be None, in which case there's no
    #  object to format and output
    # if field_name is not None, it is looked up, formatted
    #  with format_spec and conversion and then used
    def parse(self, format_string):
        return _string.formatter_parser(format_string)
    # given a field_name, find the object it references.
    #  field_name:   the field being looked up, e.g. "0.name"
    #                 or "lookup[3]"
    #  used_args:    a set of which args have been used
    #  args, kwargs: as passed in to vformat
    def get_field(self, field_name, args, kwargs):
        first, rest = _string.formatter_field_name_split(field_name)
        obj = self.get_value(first, args, kwargs)
        # loop through the rest of the field_name, doing
        #  getattr or getitem as needed
        for is_attr, i in rest:
            if is_attr:
                obj = getattr(obj, i)
            else:
                obj = obj[i]
        return obj, first