throw error when conversion required (#45078)

itazap · web-flow · commit cd5bcad54032 · 2026-04-20T10:47:57.000Z
* throw error when conversion required

* fix

* typo

* comment

* add test without sentencepiece and add PreTrainedTokenizerFast to class list
diff --git a/src/transformers/models/auto/tokenization_auto.py b/src/transformers/models/auto/tokenization_auto.py
@@ -704,8 +704,8 @@ def from_pretrained(
             else:
                 tokenizer_auto_map = tokenizer_config["auto_map"].get("AutoTokenizer", None)
 
-        # if there is a config, we can check that the tokenizer class != than model class and can thus assume we need to use TokenizersBackend
-        # Skip this early exit if auto_map is present (custom tokenizer with trust_remote_code)
+        # if there is a config, we can check that the tokenizer class != than model class.
+        # Use the config class if it's a specialized tokenizer, otherwise fall back to TokenizersBackend.
         if (
             tokenizer_auto_map is None
             and tokenizer_config_class is not None
@@ -715,15 +715,20 @@ def from_pretrained(
             and (TOKENIZER_MAPPING_NAMES.get(config_model_type).removesuffix("Fast"))
             != (tokenizer_config_class.removesuffix("Fast"))
         ):
-            # new model, but we ignore it unless the model type is the same
+            tokenizer_class = tokenizer_class_from_name(tokenizer_config_class)
+            if tokenizer_class is not None and tokenizer_class.__name__ not in (
+                "TokenizersBackend",
+                "PythonBackend",
+                "PreTrainedTokenizerFast",
+            ):
+                return tokenizer_class.from_pretrained(pretrained_model_name_or_path, *inputs, **kwargs)
+
             if TokenizersBackend is not None:
-                try:
-                    return TokenizersBackend.from_pretrained(pretrained_model_name_or_path, *inputs, **kwargs)
-                except Exception as e:
-                    logger.debug(f"Failed to use TokenizersBackend: {e}")
+                return TokenizersBackend.from_pretrained(pretrained_model_name_or_path, *inputs, **kwargs)
 
-            return tokenizer_class_from_name(tokenizer_config_class).from_pretrained(
-                pretrained_model_name_or_path, *inputs, **kwargs
+            raise ValueError(
+                f"Tokenizer class '{tokenizer_config_class}' specified in the tokenizer config was not found. "
+                f"The tokenizer may need to be converted or re-saved."
             )
 
         if "_commit_hash" in tokenizer_config:
diff --git a/tests/models/auto/test_tokenization_auto.py b/tests/models/auto/test_tokenization_auto.py
@@ -58,6 +58,7 @@
     SMALL_MODEL_IDENTIFIER,
     CaptureLogger,
     RequestCounter,
+    require_sentencepiece,
     require_tokenizers,
     slow,
 )
@@ -746,3 +747,21 @@ def __init__(self, **kwargs):
                 self.assertTrue(tokenizer.special_attribute_present)
             finally:
                 os.chdir(prev_dir)
+
+    @require_tokenizers
+    @require_sentencepiece
+    def test_mismatched_model_type_uses_config_tokenizer_class_with_sentencepiece(self):
+        tokenizer = AutoTokenizer.from_pretrained(
+            "facebook/nllb-200-distilled-600M",
+            revision="f8d333a098d19b4fd9a8b18f94170487ad3f821d",
+        )
+        self.assertEqual(tokenizer.__class__.__name__, "NllbTokenizer")
+
+    @require_tokenizers
+    def test_mismatched_model_type_uses_config_tokenizer_class_without_sentencepiece(self):
+        with mock.patch("transformers.models.auto.tokenization_auto.is_sentencepiece_available", return_value=False):
+            tokenizer = AutoTokenizer.from_pretrained(
+                "facebook/nllb-200-distilled-600M",
+                revision="f8d333a098d19b4fd9a8b18f94170487ad3f821d",
+            )
+            self.assertEqual(tokenizer.__class__.__name__, "NllbTokenizer")
