def test_save_slow_from_fast_and_reload_fast(self):
        if not self.test_slow_tokenizer or not self.test_rust_tokenizer:
            # we need both slow and fast versions
            return
    
        for tokenizer, pretrained_name, kwargs in self.tokenizers_list:
            with self.subTest(f"{tokenizer.__class__.__name__} ({pretrained_name})"):
                with tempfile.TemporaryDirectory() as tmp_dir_1:
                    # Here we check that even if we have initialized a fast tokenizer with a tokenizer_file we can
                    # still save only the slow version and use these saved files to rebuild a tokenizer
                    tokenizer_fast_old_1 = self.rust_tokenizer_class.from_pretrained(
                        pretrained_name, **kwargs, use_fast=True
                    )
                    tokenizer_file = os.path.join(tmp_dir_1, "tokenizer.json")
>                   tokenizer_fast_old_1.backend_tokenizer.save(tokenizer_file)
E                   Exception: Custom PreTokenizer cannot be serialized

src/transformers/tokenization_utils_base.py:1783: in from_pretrained
    return cls._from_pretrained(
src/transformers/tokenization_utils_base.py:1809: in _from_pretrained
    slow_tokenizer = (cls.slow_tokenizer_class)._from_pretrained(
src/transformers/tokenization_utils_base.py:1918: in _from_pretrained
    tokenizer = cls(*init_inputs, **init_kwargs)
src/transformers/models/roformer/tokenization_roformer.py:145: in __init__
    if not os.path.isfile(vocab_file):
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

path = None

    def isfile(path):
        """Test whether a path is a regular file"""
        try:
>           st = os.stat(path)
E           TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType

RUN_SLOW=1 pytest tests/roformer/test_tokenization_roformer.py -s