Easily train a new fast tokenizer from a given one (#12361)

* [WIP] Easily train a new fast tokenizer from a given one

* Fix test

* Roll out to other tokenizers and add tests

* Fix bug with unk id and add emoji to test

* Really use something different in test

* Implement special tokens map

* Map special tokens in the Transformers tokenizers

* Fix test

* Make test more robust

* Fix test for BPE

* More robust map and test

Co-authored-by SaulLu

* Test file

* Stronger tests

Co-authored-by: SaulLu <lucilesaul.com@gmail.com>

* Map unk token for Wordpiece and address review comment

* Fix lowercase test and address review comment

* Fix all tests

* Simplify test

* Fix tests for realsies

* Easily train a new fast tokenizer from a given one - tackle the special tokens format (str or AddedToken) (#12420)

* Propose change in tests regarding lower case

* add new test for special tokens types

* put back the test part about decoding

* add feature: the AddedToken is re-build with the different mapped content

* Address review comment: simplify AddedToken building

Co-authored-by: sgugger <sylvain.gugger@gmail.com>

* Update src/transformers/tokenization_utils_fast.py

Co-authored-by: sgugger <sylvain.gugger@gmail.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

Co-authored-by: SaulLu <lucilesaul.com@gmail.com>
Co-authored-by: SaulLu <55560583+SaulLu@users.noreply.github.com>

src/transformers/models/albert/tokenization_albert_fast.py

@@ -121,7 +121,7 @@ class AlbertTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         do_lower_case=True,
         remove_space=True,

src/transformers/models/barthez/tokenization_barthez_fast.py

@@ -109,7 +109,7 @@ class BarthezTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         bos_token="<s>",
         eos_token="</s>",

src/transformers/models/bert/tokenization_bert_fast.py

@@ -162,7 +162,7 @@ class BertTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         do_lower_case=True,
         unk_token="[UNK]",

src/transformers/models/big_bird/tokenization_big_bird_fast.py

@@ -103,7 +103,7 @@ class BigBirdTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         unk_token="<unk>",
         bos_token="<s>",

src/transformers/models/blenderbot_small/tokenization_blenderbot_small_fast.py

@@ -63,8 +63,8 @@ class BlenderbotSmallTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
-        merges_file,
+        vocab_file=None,
+        merges_file=None,
         unk_token="<|endoftext|>",
         bos_token="<|endoftext|>",
         eos_token="<|endoftext|>",

src/transformers/models/camembert/tokenization_camembert_fast.py

@@ -105,7 +105,7 @@ class CamembertTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         bos_token="<s>",
         eos_token="</s>",

src/transformers/models/clip/tokenization_clip_fast.py

@@ -105,8 +105,8 @@ class CLIPTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
-        merges_file,
+        vocab_file=None,
+        merges_file=None,
         tokenizer_file=None,
         unk_token="<|endoftext|>",
         bos_token="<|startoftext|>",

src/transformers/models/deberta/tokenization_deberta_fast.py

@@ -95,8 +95,8 @@ class DebertaTokenizerFast(GPT2TokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
-        merges_file,
+        vocab_file=None,
+        merges_file=None,
         tokenizer_file=None,
         errors="replace",
         bos_token="[CLS]",

src/transformers/models/funnel/tokenization_funnel_fast.py

@@ -88,7 +88,7 @@ class FunnelTokenizerFast(BertTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         do_lower_case=True,
         unk_token="<unk>",

src/transformers/models/gpt2/tokenization_gpt2_fast.py

@@ -125,8 +125,8 @@ class GPT2TokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
-        merges_file,
+        vocab_file=None,
+        merges_file=None,
         tokenizer_file=None,
         unk_token="<|endoftext|>",
         bos_token="<|endoftext|>",

src/transformers/models/herbert/tokenization_herbert_fast.py

@@ -67,8 +67,8 @@ class HerbertTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
-        merges_file,
+        vocab_file=None,
+        merges_file=None,
         tokenizer_file=None,
         cls_token="<s>",
         unk_token="<unk>",

src/transformers/models/mbart/tokenization_mbart.py

@@ -121,7 +121,10 @@ class MBartTokenizer(XLMRobertaTokenizer):
         self._additional_special_tokens = list(self.lang_code_to_id.keys())
 
         if additional_special_tokens is not None:
-            self._additional_special_tokens.extend(additional_special_tokens)
+            # Only add those special tokens if they are not already there.
+            self._additional_special_tokens.extend(
+                [t for t in additional_special_tokens if t not in self._additional_special_tokens]
+            )
 
         self._src_lang = src_lang if src_lang is not None else "en_XX"
         self.cur_lang_code_id = self.lang_code_to_id[self._src_lang]

src/transformers/models/mbart/tokenization_mbart50_fast.py

@@ -110,7 +110,7 @@ class MBart50TokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         src_lang=None,
         tgt_lang=None,
         tokenizer_file=None,

src/transformers/models/mbart/tokenization_mbart_fast.py

@@ -113,10 +113,16 @@ class MBartTokenizerFast(XLMRobertaTokenizerFast):
     suffix_tokens: List[int] = []
 
     def __init__(
-        self, *args, tokenizer_file=None, src_lang=None, tgt_lang=None, additional_special_tokens=None, **kwargs
+        self,
+        vocab_file=None,
+        tokenizer_file=None,
+        src_lang=None,
+        tgt_lang=None,
+        additional_special_tokens=None,
+        **kwargs
     ):
         super().__init__(
-            *args,
+            vocab_file=vocab_file,
             tokenizer_file=tokenizer_file,
             src_lang=src_lang,
             tgt_lang=tgt_lang,
@@ -127,7 +133,10 @@ class MBartTokenizerFast(XLMRobertaTokenizerFast):
         _additional_special_tokens = FAIRSEQ_LANGUAGE_CODES.copy()
 
         if additional_special_tokens is not None:
-            _additional_special_tokens.extend(additional_special_tokens)
+            # Only add those special tokens if they are not already there.
+            _additional_special_tokens.extend(
+                [t for t in additional_special_tokens if t not in _additional_special_tokens]
+            )
 
         self.add_special_tokens({"additional_special_tokens": _additional_special_tokens})
 

src/transformers/models/mpnet/tokenization_mpnet_fast.py

@@ -106,7 +106,7 @@ class MPNetTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         do_lower_case=True,
         bos_token="<s>",

src/transformers/models/openai/tokenization_openai_fast.py

@@ -64,7 +64,7 @@ class OpenAIGPTTokenizerFast(PreTrainedTokenizerFast):
     model_input_names = ["input_ids", "attention_mask"]
     slow_tokenizer_class = OpenAIGPTTokenizer
 
-    def __init__(self, vocab_file, merges_file, tokenizer_file=None, unk_token="<unk>", **kwargs):
+    def __init__(self, vocab_file=None, merges_file=None, tokenizer_file=None, unk_token="<unk>", **kwargs):
         super().__init__(vocab_file, merges_file, tokenizer_file=tokenizer_file, unk_token=unk_token, **kwargs)
 
     @property

src/transformers/models/pegasus/tokenization_pegasus_fast.py

@@ -98,7 +98,7 @@ class PegasusTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         pad_token="<pad>",
         eos_token="</s>",

src/transformers/models/reformer/tokenization_reformer_fast.py

@@ -87,7 +87,7 @@ class ReformerTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         eos_token="</s>",
         unk_token="<unk>",

src/transformers/models/roberta/tokenization_roberta_fast.py

@@ -143,8 +143,8 @@ class RobertaTokenizerFast(GPT2TokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
-        merges_file,
+        vocab_file=None,
+        merges_file=None,
         tokenizer_file=None,
         errors="replace",
         bos_token="<s>",

src/transformers/models/roformer/tokenization_roformer_fast.py

@@ -73,7 +73,7 @@ class RoFormerTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         do_lower_case=True,
         unk_token="[UNK]",

src/transformers/models/t5/tokenization_t5_fast.py

@@ -104,7 +104,7 @@ class T5TokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         eos_token="</s>",
         unk_token="<unk>",

src/transformers/models/xlm_roberta/tokenization_xlm_roberta_fast.py

@@ -117,7 +117,7 @@ class XLMRobertaTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         bos_token="<s>",
         eos_token="</s>",

src/transformers/models/xlnet/tokenization_xlnet_fast.py

@@ -124,7 +124,7 @@ class XLNetTokenizerFast(PreTrainedTokenizerFast):
 
     def __init__(
         self,
-        vocab_file,
+        vocab_file=None,
         tokenizer_file=None,
         do_lower_case=False,
         remove_space=True,

src/transformers/tokenization_utils_fast.py

@@ -16,7 +16,6 @@
  Tokenization classes for fast tokenizers (provided by HuggingFace's tokenizers library). For slow (python) tokenizers
  see tokenization_utils.py
 """
-
 import json
 import os
 from collections import defaultdict
@@ -25,6 +24,7 @@ from typing import Any, Dict, List, Optional, Tuple, Union
 from tokenizers import Encoding as EncodingFast
 from tokenizers import Tokenizer as TokenizerFast
 from tokenizers.decoders import Decoder as DecoderFast
+from tokenizers.trainers import BpeTrainer, UnigramTrainer, WordLevelTrainer, WordPieceTrainer
 
 from .convert_slow_tokenizer import convert_slow_tokenizer
 from .file_utils import PaddingStrategy, add_end_docstrings
@@ -36,6 +36,7 @@ from .tokenization_utils_base import (
     PreTokenizedInput,
     PreTokenizedInputPair,
     PreTrainedTokenizerBase,
+    SpecialTokensMixin,
     TextInput,
     TextInputPair,
     TruncationStrategy,
@@ -60,6 +61,13 @@ INIT_TOKENIZER_DOCSTRING += """
             from 🤗 tokenizers <../fast_tokenizers>` for more information.
 """
 
+MODEL_TO_TRAINER_MAPPING = {
+    "BPE": BpeTrainer,
+    "Unigram": UnigramTrainer,
+    "WordLevel": WordLevelTrainer,
+    "WordPiece": WordPieceTrainer,
+}
+
 
 @add_end_docstrings(INIT_TOKENIZER_DOCSTRING)
 class PreTrainedTokenizerFast(PreTrainedTokenizerBase):
@@ -555,3 +563,162 @@ class PreTrainedTokenizerFast(PreTrainedTokenizerBase):
             file_names = file_names + (tokenizer_file,)
 
         return file_names
+
+    def train_new_from_iterator(
+        self, text_iterator, vocab_size, new_special_tokens=None, special_tokens_map=None, **kwargs
+    ):
+        """
+        Trains a tokenizer on a new corpus with the same defaults (in terms of special tokens or tokenization pipeline)
+        as the current one.
+
+        Args:
+            text_iterator (generator of :obj:`List[str]`):
+                The training corpus. Should be a generator of batches of texts, for instance a list of lists of texts
+                if you have everything in memory.
+            vocab_size (obj:`int`):
+                The size of the vocabulary you want for your tokenizer.
+            new_special_tokens (list of :obj:`str` or :obj:`AddedToken`, `optional`):
+                A list of new special tokens to add to the tokenizer you are training.
+            special_tokens_map (:obj:`Dict[str, str]`, `optional`):
+                If you want to rename some of the special tokens this tokenizer uses, pass along a mapping old special
+                token name to new special token name in this argument.
+            kwargs:
+                Additional keyword arguments passed along to the trainer from the 🤗 Tokenizers library.
+
+        Returns:
+            :class:`~transformers.PreTrainedTokenizerFast`: A new tokenizer of the same type as the original one,
+            trained on :obj:`text_iterator`.
+
+        """
+        tokenizer_json = json.loads(self._tokenizer.to_str())
+        # Remove added tokens for now (uses IDs of tokens)
+        added_tokens = tokenizer_json.pop("added_tokens")
+        # Remove post processor for now (uses IDs of tokens)
+        post_processor = tokenizer_json.pop("post_processor")
+
+        unk_token = None
+        # Remove vocab
+        if tokenizer_json["model"]["type"] == "BPE":
+            tokenizer_json["model"]["vocab"] = {}
+            tokenizer_json["model"]["merges"] = []
+        elif tokenizer_json["model"]["type"] == "Unigram":
+            if tokenizer_json["model"]["unk_id"] is not None:
+                unk_id = tokenizer_json["model"]["unk_id"]
+                unk_token = tokenizer_json["model"]["vocab"][unk_id][0]
+                if special_tokens_map is not None and unk_token in special_tokens_map:
+                    unk_token = special_tokens_map[unk_token]
+                tokenizer_json["model"]["unk_id"] = 0
+                tokenizer_json["model"]["vocab"] = [[unk_token, 0.0]]
+        elif tokenizer_json["model"]["type"] in ["WordLevel", "WordPiece"]:
+            tokenizer_json["model"]["vocab"] = {}
+        else:
+            raise ValueError(
+                f"This method does not support this type of tokenizer (found {tokenizer_json['model']['type']}) "
+                "only BPE, Unigram, WordLevel and WordPiece."
+            )
+
+        if (
+            special_tokens_map is not None
+            and "unk_token" in tokenizer_json["model"]
+            and tokenizer_json["model"]["unk_token"] in special_tokens_map
+        ):
+            tokenizer_json["model"]["unk_token"] = special_tokens_map[tokenizer_json["model"]["unk_token"]]
+
+        tokenizer = TokenizerFast.from_str(json.dumps(tokenizer_json))
+
+        # Get the special tokens from the current tokenizer if none are specified.
+        special_tokens = []
+        for added_token in added_tokens:
+            special = added_token.pop("special", None)
+            _ = added_token.pop("id", None)
+            if tokenizer_json["model"]["type"] != "Unigram" and not special:
+                continue
+            if special_tokens_map is not None and added_token["content"] in special_tokens_map:
+                added_token["content"] = special_tokens_map[added_token["content"]]
+            special_tokens.append(AddedToken(**added_token))
+
+        if new_special_tokens is not None:
+            special_tokens.extend(new_special_tokens)
+
+        # Trainer needs to know the end of word / continuing subword thingies in BPE
+        if (
+            tokenizer_json["model"]["type"] == "BPE"
+            and "continuing_subword_prefix" not in kwargs
+            and tokenizer_json["model"]["continuing_subword_prefix"] is not None
+        ):
+            kwargs["continuing_subword_prefix"] = tokenizer_json["model"]["continuing_subword_prefix"]
+        if (
+            tokenizer_json["model"]["type"] == "BPE"
+            and "end_of_work_suffix" not in kwargs
+            and tokenizer_json["model"]["end_of_word_suffix"] is not None
+        ):
+            kwargs["end_of_word_suffix"] = tokenizer_json["model"]["end_of_word_suffix"]
+
+        trainer_class = MODEL_TO_TRAINER_MAPPING[tokenizer_json["model"]["type"]]
+        trainer = trainer_class(vocab_size=vocab_size, special_tokens=special_tokens, **kwargs)
+        tokenizer.train_from_iterator(text_iterator, trainer=trainer)
+
+        if unk_token is not None:
+            # For Unigram tokenizers we need to set back the unk id of the model (bug in Tokenizers?)
+            trained_tokenizer_json = json.loads(tokenizer.to_str())
+            vocab = trained_tokenizer_json["model"]["vocab"]
+            unk_id = 0
+            while unk_id < len(vocab) and vocab[unk_id][0] != unk_token:
+                unk_id += 1
+            if unk_id < len(vocab):
+                trained_tokenizer_json["model"]["unk_id"] = unk_id
+                tokenizer = TokenizerFast.from_str(json.dumps(trained_tokenizer_json))
+
+        if post_processor is not None:
+            trained_tokenizer_json = json.loads(tokenizer.to_str())
+            # Almost done, we just have to adjust the token IDs in the post processor
+            if "special_tokens" in post_processor:
+                for key in post_processor["special_tokens"]:
+                    tokens = post_processor["special_tokens"][key]["tokens"]
+                    if special_tokens_map is not None:
+                        tokens = [special_tokens_map.get(token, token) for token in tokens]
+                    post_processor["special_tokens"][key]["tokens"] = tokens
+                    post_processor["special_tokens"][key]["ids"] = [tokenizer.token_to_id(token) for token in tokens]
+
+            for special_token in ["cls", "sep"]:
+                if special_token in post_processor:
+                    token, _ = post_processor[special_token]
+                    if special_tokens_map is not None and token in special_tokens_map:
+                        token = special_tokens_map[token]
+                    token_id = tokenizer.token_to_id(token)
+                    post_processor[special_token] = [token, token_id]
+
+            trained_tokenizer_json["post_processor"] = post_processor
+            tokenizer = TokenizerFast.from_str(json.dumps(trained_tokenizer_json))
+
+        kwargs = self.init_kwargs.copy()
+        # Map pad/cls/mask token at the Transformers level
+        special_tokens_list = SpecialTokensMixin.SPECIAL_TOKENS_ATTRIBUTES.copy()
+        special_tokens_list.remove("additional_special_tokens")
+        for token in special_tokens_list:
+            # Get the private one to avoid unnecessary warnings.
+            if getattr(self, f"_{token}") is not None:
+                special_token = getattr(self, token)
+                if special_tokens_map is not None and special_token in special_tokens_map:
+                    special_token = special_tokens_map[special_token]
+
+                special_token_full = getattr(self, f"_{token}")
+                if isinstance(special_token_full, AddedToken):
+                    # Create an added token with the same paramters except the content
+                    kwargs[token] = AddedToken(
+                        special_token,
+                        single_word=special_token_full.single_word,
+                        lstrip=special_token_full.lstrip,
+                        rstrip=special_token_full.rstrip,
+                        normalized=special_token_full.normalized,
+                    )
+                else:
+                    kwargs[token] = special_token
+
+        additional_special_tokens = self.additional_special_tokens
+        if new_special_tokens is not None:
+            additional_special_tokens.extend(new_special_tokens)
+        if len(additional_special_tokens) > 0:
+            kwargs["additional_special_tokens"] = additional_special_tokens
+
+        return self.__class__(tokenizer_object=tokenizer, **kwargs)

tests/test_tokenization_common.py

@@ -33,6 +33,7 @@ from transformers import (
     PreTrainedTokenizer,
     PreTrainedTokenizerBase,
     PreTrainedTokenizerFast,
+    SpecialTokensMixin,
     is_tf_available,
     is_torch_available,
 )
@@ -57,6 +58,11 @@ if TYPE_CHECKING:
 
 NON_ENGLISH_TAGS = ["chinese", "dutch", "french", "finnish", "german", "multilingual"]
 
+SMALL_TRAINING_CORPUS = [
+    ["This is the first sentence.", "This is the second one."],
+    ["This sentence (contains #) over symbols and numbers 12 3.", "But not this one."],
+]
+
 
 def filter_non_english(_, pretrained_name: str):
     """Filter all the model for non-english language"""
@@ -390,7 +396,11 @@ class TokenizerTesterMixin:
         tokenizer = self.get_rust_tokenizer()
 
         for parameter_name, parameter in signature.parameters.items():
-            if parameter.default != inspect.Parameter.empty and parameter_name != "tokenizer_file":
+            if parameter.default != inspect.Parameter.empty and parameter_name not in [
+                "vocab_file",
+                "merges_file",
+                "tokenizer_file",
+            ]:
                 self.assertIn(parameter_name, tokenizer.init_kwargs)
 
     def test_rust_and_python_full_tokenizers(self):
@@ -3144,6 +3154,146 @@ class TokenizerTesterMixin:
                     self.assertTrue(special_token_id in p_output)
                     self.assertTrue(special_token_id in cr_output)
 
+    def test_training_new_tokenizer(self):
+        # This feature only exists for fast tokenizers
+        if not self.test_rust_tokenizer:
+            return
+
+        tokenizer = self.get_rust_tokenizer()
+        new_tokenizer = tokenizer.train_new_from_iterator(SMALL_TRAINING_CORPUS, 100)
+
+        # Test we can use the new tokenizer with something not seen during training
+        inputs = new_tokenizer(["This is the first sentence", "This sentence is different 🤗."])
+        self.assertEqual(len(inputs["input_ids"]), 2)
+        decoded_input = new_tokenizer.decode(inputs["input_ids"][0], skip_special_tokens=True)
+        expected_result = "This is the first sentence"
+
+        # OpenAIGPT always lowercases and has no arg.
+        if new_tokenizer.init_kwargs.get("do_lower_case", False) or tokenizer.__class__.__name__.startswith(
+            "OpenAIGPT"
+        ):
+            expected_result = expected_result.lower()
+        self.assertEqual(expected_result, decoded_input)
+
+        # We check that the parameters of the tokenizer remained the same
+        # Check we have the same number of added_tokens for both pair and non-pair inputs.
+        self.assertEqual(tokenizer.num_special_tokens_to_add(False), new_tokenizer.num_special_tokens_to_add(False))
+        self.assertEqual(tokenizer.num_special_tokens_to_add(True), new_tokenizer.num_special_tokens_to_add(True))
+
+        # Check we have the correct max_length for both pair and non-pair inputs.
+        self.assertEqual(tokenizer.max_len_single_sentence, new_tokenizer.max_len_single_sentence)
+        self.assertEqual(tokenizer.max_len_sentences_pair, new_tokenizer.max_len_sentences_pair)
+
+        # Assert the set of special tokens match as we didn't ask to change them
+        self.assertSequenceEqual(
+            tokenizer.all_special_tokens_extended,
+            new_tokenizer.all_special_tokens_extended,
+        )
+
+        self.assertDictEqual(tokenizer.special_tokens_map, new_tokenizer.special_tokens_map)
+
+    def test_training_new_tokenizer_with_special_tokens_change(self):
+        # This feature only exists for fast tokenizers
+        if not self.test_rust_tokenizer:
+            return
+
+        tokenizer = self.get_rust_tokenizer()
+        # Test with a special tokens map
+        class_signature = inspect.signature(tokenizer.__class__)
+        if "cls_token" in class_signature.parameters:
+            new_tokenizer = tokenizer.train_new_from_iterator(
+                SMALL_TRAINING_CORPUS, 100, special_tokens_map={tokenizer.cls_token: "<cls>"}
+            )
+            cls_id = new_tokenizer.get_vocab()["<cls>"]
+            self.assertEqual(new_tokenizer.cls_token, "<cls>")
+            self.assertEqual(new_tokenizer.cls_token_id, cls_id)
+
+        # Create a new mapping from the special tokens defined in the original tokenizer
+        special_tokens_list = SpecialTokensMixin.SPECIAL_TOKENS_ATTRIBUTES.copy()
+        special_tokens_list.remove("additional_special_tokens")
+        special_tokens_map = {}
+        for token in special_tokens_list:
+            # Get the private one to avoid unnecessary warnings.
+            if getattr(tokenizer, f"_{token}") is not None:
+                special_token = getattr(tokenizer, token)
+                special_tokens_map[special_token] = f"{special_token}a"
+
+        # Train new tokenizer
+        new_tokenizer = tokenizer.train_new_from_iterator(
+            SMALL_TRAINING_CORPUS, 100, special_tokens_map=special_tokens_map
+        )
+
+        # Check the changes
+        for token in special_tokens_list:
+            # Get the private one to avoid unnecessary warnings.
+            if getattr(tokenizer, f"_{token}") is None:
+                continue
+            special_token = getattr(tokenizer, token)
+            if special_token in special_tokens_map:
+                new_special_token = getattr(new_tokenizer, token)
+                self.assertEqual(special_tokens_map[special_token], new_special_token)
+
+                new_id = new_tokenizer.get_vocab()[new_special_token]
+                self.assertEqual(getattr(new_tokenizer, f"{token}_id"), new_id)
+
+        # Check if the AddedToken / string format has been kept
+        for special_token in tokenizer.all_special_tokens_extended:
+            if isinstance(special_token, AddedToken) and special_token.content not in special_tokens_map:
+                # The special token must appear identically in the list of the new tokenizer.
+                self.assertTrue(
+                    special_token in new_tokenizer.all_special_tokens_extended,
+                    f"'{special_token}' should be in {new_tokenizer.all_special_tokens_extended}",
+                )
+            elif isinstance(special_token, AddedToken):
+                # The special token must appear in the list of the new tokenizer as an object of type AddedToken with
+                # the same parameters as the old AddedToken except the content that the user has requested to change.
+                special_token_str = special_token.content
+                new_special_token_str = special_tokens_map[special_token_str]
+
+                find = False
+                for candidate in new_tokenizer.all_special_tokens_extended:
+                    if (
+                        isinstance(candidate, AddedToken)
+                        and candidate.content == new_special_token_str
+                        and candidate.lstrip == special_token.lstrip
+                        and candidate.rstrip == special_token.rstrip
+                        and candidate.normalized == special_token.normalized
+                        and candidate.single_word == special_token.single_word
+                    ):
+                        find = True
+                        break
+                self.assertTrue(
+                    find,
+                    (
+                        f"'{new_special_token_str}' doesn't appear in the list "
+                        f"'{new_tokenizer.all_special_tokens_extended}' as an AddedToken with the same parameters as "
+                        f"'{special_token}' in the list {tokenizer.all_special_tokens_extended}"
+                    ),
+                )
+            elif special_token not in special_tokens_map:
+                # The special token must appear identically in the list of the new tokenizer.
+                self.assertTrue(
+                    special_token in new_tokenizer.all_special_tokens_extended,
+                    f"'{special_token}' should be in {new_tokenizer.all_special_tokens_extended}",
+                )
+
+            else:
+                # The special token must appear in the list of the new tokenizer as an object of type string.
+                self.assertTrue(special_tokens_map[special_token] in new_tokenizer.all_special_tokens_extended)
+
+        # Test we can use the new tokenizer with something not seen during training
+        inputs = new_tokenizer(["This is the first sentence", "This sentence is different 🤗."])
+        self.assertEqual(len(inputs["input_ids"]), 2)
+        decoded_input = new_tokenizer.decode(inputs["input_ids"][0], skip_special_tokens=True)
+        expected_result = "This is the first sentence"
+
+        # OpenAIGPT always lowercases and has no arg.
+        if new_tokenizer.init_kwargs.get("do_lower_case", False) or tokenizer.__class__.__name__.startswith(
+            "OpenAIGPT"
+        ):
+            expected_result = expected_result.lower()
+        self.assertEqual(expected_result, decoded_input)
+
 
 @is_staging_test
 class TokenizerPushToHubTester(unittest.TestCase):