def format_field(self, value, format_spec):
    if isinstance(value, (list, tuple, set, frozenset)):
        return self.separator.join((self.format_element(v, format_spec) for v in value))
    else:
        return self.format_element(value, format_spec)
def format_field(self, value, format_spec):
    if isinstance(value, (list, tuple, set, frozenset)):
        return self.separator.join((self.format_element(v, format_spec) for v in value))
    else:
        return self.format_element(value, format_spec)