src/transformers/tokenization_utils_base.py/SpecialTokensMixin/all_special_tokens_extended
class SpecialTokensMixin:    def all_special_tokens_extended(self) -> List[Union[str, AddedToken]]:
        """
        :obj:`List[Union[str, tokenizers.AddedToken]]`: All the special tokens (:obj:`'<unk>'`, :obj:`'<cls>'`, etc.)
        mapped to class attributes.

        Don't convert tokens of :obj:`tokenizers.AddedToken` type to string so they can be used to control more finely
        how special tokens are tokenized.
        """
        all_toks = []
        set_attr = self.special_tokens_map_extended
        for attr_value in set_attr.values():
            all_toks = all_toks + (list(attr_value) if isinstance(attr_value, (list, tuple)) else [attr_value])
        all_toks = list(OrderedDict.fromkeys(all_toks))
        return all_toks