src/transformers/tokenization_utils_base.py/SpecialTokensMixin/__init__
class SpecialTokensMixin:    def __init__(self, verbose=True, **kwargs):
        self._bos_token = None
        self._eos_token = None
        self._unk_token = None
        self._sep_token = None
        self._pad_token = None
        self._cls_token = None
        self._mask_token = None
        self._pad_token_type_id = 0
        self._additional_special_tokens = []
        self.verbose = verbose

        # We directly set the hidden value to allow initialization with special tokens
        # which are not yet in the vocabulary. Necessary for serialization/de-serialization
        # TODO clean this up at some point (probably by switching to fast tokenizers)
        for key, value in kwargs.items():
            if value is None:
                continue
            if key in self.SPECIAL_TOKENS_ATTRIBUTES:
                if key == "additional_special_tokens":
                    assert isinstance(value, (list, tuple)), f"Value {value} is not a list or tuple"
                    assert all(
                        isinstance(t, (str, AddedToken)) for t in value
                    ), "One of the tokens is not a string or an AddedToken"
                    setattr(self, key, value)
                elif isinstance(value, (str, AddedToken)):
                    setattr(self, key, value)
                else:
                    raise TypeError(f"special token {key} has to be either str or AddedToken but got: {type(value)}")