sum() is the leanest method to flatten a string list, so it's been replaced by itertools.chain.from_iterable()

transformers/tokenization_utils.py

@@ -21,6 +21,7 @@ import os
 import json
 import six
 import copy
+import itertools
 from io import open
 
 from .file_utils import cached_path, is_tf_available, is_torch_available
@@ -641,9 +642,9 @@ class PreTrainedTokenizer(object):
                         tokenized_text += [sub_text]
                 text_list = tokenized_text
 
-            return sum((self._tokenize(token, **kwargs) if token not \
+            return list(itertools.chain.from_iterable((self._tokenize(token, **kwargs) if token not \
                     in self.added_tokens_encoder and token not in self.all_special_tokens \
-                    else [token] for token in tokenized_text), [])
+                    else [token] for token in tokenized_text)))
 
         added_tokens = list(self.added_tokens_encoder.keys()) + self.all_special_tokens
         tokenized_text = split_on_tokens(added_tokens, text)