diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index bb9459ecb3..a48c476989 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -198,11 +198,12 @@ Follow these steps to start contributing:
    are useful to avoid duplicated work, and to differentiate it from PRs ready
    to be merged;
 4. Make sure existing tests pass;
-5. Add high-coverage tests. No quality test, no merge. 
+5. Add high-coverage tests. No quality testing = no merge. 
  - If you are adding a new model, make sure that you use `ModelTester.all_model_classes = (MyModel, MyModelWithLMHead,...)`, which triggers the common tests.
  - If you are adding new `@slow` tests, make sure they pass using `RUN_SLOW=1 python -m pytest tests/test_my_new_model.py`. 
+ - If you are adding a new tokenizer, write tests, and make sure `RUN_SLOW=1 python -m pytest tests/test_tokenization_{your_model_name}.py` passes.
 CircleCI does not run them. 
-6. All public methods must have informative docstrings;
+6. All public methods must have informative docstrings that work nicely with sphinx. See `modeling_ctrl.py` for an example.
 
 ### Tests
 
diff --git a/src/transformers/tokenization_roberta.py b/src/transformers/tokenization_roberta.py
index 6e05f6e60c..46cedcefa7 100644
--- a/src/transformers/tokenization_roberta.py
+++ b/src/transformers/tokenization_roberta.py
@@ -199,7 +199,7 @@ class RobertaTokenizer(GPT2Tokenizer):
             if token_ids_1 is not None:
                 raise ValueError(
                     "You should not supply a second sequence if the provided sequence of "
-                    "ids is already formated with special tokens for the model."
+                    "ids is already formatted with special tokens for the model."
                 )
             return list(map(lambda x: 1 if x in [self.sep_token_id, self.cls_token_id] else 0, token_ids_0))
 
diff --git a/src/transformers/tokenization_utils.py b/src/transformers/tokenization_utils.py
index 5eba847bd0..5e8d7df009 100644
--- a/src/transformers/tokenization_utils.py
+++ b/src/transformers/tokenization_utils.py
@@ -771,26 +771,26 @@ class PreTrainedTokenizer(SpecialTokensMixin):
         raise NotImplementedError
 
     @property
-    def is_fast(self):
+    def is_fast(self) -> bool:
         return False
 
     @property
-    def max_len(self):
+    def max_len(self) -> int:
         """ Kept here for backward compatibility.
             Now renamed to `model_max_length` to avoid ambiguity.
         """
         return self.model_max_length
 
     @property
-    def max_len_single_sentence(self):
+    def max_len_single_sentence(self) -> int:
         return self.model_max_length - self.num_special_tokens_to_add(pair=False)
 
     @property
-    def max_len_sentences_pair(self):
+    def max_len_sentences_pair(self) -> int:
         return self.model_max_length - self.num_special_tokens_to_add(pair=True)
 
     @max_len_single_sentence.setter
-    def max_len_single_sentence(self, value):
+    def max_len_single_sentence(self, value) -> int:
         """ For backward compatibility, allow to try to setup 'max_len_single_sentence' """
         if value == self.model_max_length - self.num_special_tokens_to_add(pair=False):
             logger.warning(
@@ -802,7 +802,7 @@ class PreTrainedTokenizer(SpecialTokensMixin):
             )
 
     @max_len_sentences_pair.setter
-    def max_len_sentences_pair(self, value):
+    def max_len_sentences_pair(self, value) -> int:
         """ For backward compatibility, allow to try to setup 'max_len_sentences_pair' """
         if value == self.model_max_length - self.num_special_tokens_to_add(pair=True):
             logger.warning(
@@ -1118,7 +1118,7 @@ class PreTrainedTokenizer(SpecialTokensMixin):
 
         return vocab_files + (special_tokens_map_file, added_tokens_file)
 
-    def save_vocabulary(self, save_directory):
+    def save_vocabulary(self, save_directory) -> Tuple[str]:
         """ Save the tokenizer vocabulary to a directory. This method does *NOT* save added tokens
             and special token mappings.
 
@@ -1128,7 +1128,7 @@ class PreTrainedTokenizer(SpecialTokensMixin):
         """
         raise NotImplementedError
 
-    def add_tokens(self, new_tokens):
+    def add_tokens(self, new_tokens: Union[str, List[str]]) -> int:
         """
         Add a list of new tokens to the tokenizer class. If the new tokens are not in the
         vocabulary, they are added to it with indices starting from length of the current vocabulary.
@@ -1156,7 +1156,7 @@ class PreTrainedTokenizer(SpecialTokensMixin):
         if not isinstance(new_tokens, list):
             new_tokens = [new_tokens]
 
-        to_add_tokens = []
+        tokens_to_add = []
         for token in new_tokens:
             assert isinstance(token, str)
             if self.init_kwargs.get("do_lower_case", False) and token not in self.all_special_tokens:
@@ -1164,18 +1164,18 @@ class PreTrainedTokenizer(SpecialTokensMixin):
             if (
                 token != self.unk_token
                 and self.convert_tokens_to_ids(token) == self.convert_tokens_to_ids(self.unk_token)
-                and token not in to_add_tokens
+                and token not in tokens_to_add
             ):
-                to_add_tokens.append(token)
+                tokens_to_add.append(token)
                 logger.info("Adding %s to the vocabulary", token)
 
-        added_tok_encoder = dict((tok, len(self) + i) for i, tok in enumerate(to_add_tokens))
+        added_tok_encoder = dict((tok, len(self) + i) for i, tok in enumerate(tokens_to_add))
         added_tok_decoder = {v: k for k, v in added_tok_encoder.items()}
         self.added_tokens_encoder.update(added_tok_encoder)
         self.unique_added_tokens_encoder = set(self.added_tokens_encoder.keys()).union(set(self.all_special_tokens))
         self.added_tokens_decoder.update(added_tok_decoder)
 
-        return len(to_add_tokens)
+        return len(tokens_to_add)
 
     def num_special_tokens_to_add(self, pair=False):
         """
@@ -2080,10 +2080,7 @@ class PreTrainedTokenizer(SpecialTokensMixin):
     def build_inputs_with_special_tokens(self, token_ids_0: List, token_ids_1: Optional[List] = None) -> List:
         """
         Build model inputs from a sequence or a pair of sequence for sequence classification tasks
-        by concatenating and adding special tokens.
-        A RoBERTa sequence has the following format:
-            single sequence: <s> X </s>
-            pair of sequences: <s> A </s></s> B </s>
+        by concatenating and adding special tokens. This implementation does not add special tokens.
         """
         if token_ids_1 is None:
             return token_ids_0
diff --git a/tests/test_tokenization_albert.py b/tests/test_tokenization_albert.py
index c190d8ed82..b64a7c4b32 100644
--- a/tests/test_tokenization_albert.py
+++ b/tests/test_tokenization_albert.py
@@ -36,9 +36,6 @@ class AlbertTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
         tokenizer = AlbertTokenizer(SAMPLE_VOCAB)
         tokenizer.save_pretrained(self.tmpdirname)
 
-    def get_tokenizer(self, **kwargs):
-        return AlbertTokenizer.from_pretrained(self.tmpdirname, **kwargs)
-
     def get_input_output_texts(self):
         input_text = "this is a test"
         output_text = "this is a test"
diff --git a/tests/test_tokenization_bert.py b/tests/test_tokenization_bert.py
index 0e81eb1a5a..2a303768ad 100644
--- a/tests/test_tokenization_bert.py
+++ b/tests/test_tokenization_bert.py
@@ -59,9 +59,6 @@ class BertTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
         with open(self.vocab_file, "w", encoding="utf-8") as vocab_writer:
             vocab_writer.write("".join([x + "\n" for x in vocab_tokens]))
 
-    def get_tokenizer(self, **kwargs):
-        return BertTokenizer.from_pretrained(self.tmpdirname, **kwargs)
-
     def get_rust_tokenizer(self, **kwargs):
         return BertTokenizerFast.from_pretrained(self.tmpdirname, **kwargs)
 
diff --git a/tests/test_tokenization_bert_japanese.py b/tests/test_tokenization_bert_japanese.py
index 4e0925d729..7ff22afd7a 100644
--- a/tests/test_tokenization_bert_japanese.py
+++ b/tests/test_tokenization_bert_japanese.py
@@ -60,9 +60,6 @@ class BertJapaneseTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
         with open(self.vocab_file, "w", encoding="utf-8") as vocab_writer:
             vocab_writer.write("".join([x + "\n" for x in vocab_tokens]))
 
-    def get_tokenizer(self, **kwargs):
-        return BertJapaneseTokenizer.from_pretrained(self.tmpdirname, **kwargs)
-
     def get_input_output_texts(self):
         input_text = "こんにちは、世界。 \nこんばんは、世界。"
         output_text = "こんにちは 、 世界 。 こんばんは 、 世界 。"
diff --git a/tests/test_tokenization_common.py b/tests/test_tokenization_common.py
index acad8d655f..5bbb087192 100644
--- a/tests/test_tokenization_common.py
+++ b/tests/test_tokenization_common.py
@@ -22,12 +22,12 @@ from collections import OrderedDict
 from typing import TYPE_CHECKING, Dict, Tuple, Union
 
 from tests.utils import require_tf, require_torch
+from transformers import PreTrainedTokenizer
 
 
 if TYPE_CHECKING:
     from transformers import (
         PretrainedConfig,
-        PreTrainedTokenizer,
         PreTrainedTokenizerFast,
         PreTrainedModel,
         TFPreTrainedModel,
@@ -67,19 +67,24 @@ class TokenizerTesterMixin:
     def tearDown(self):
         shutil.rmtree(self.tmpdirname)
 
-    def get_tokenizer(self, **kwargs):
-        raise NotImplementedError
+    def get_tokenizer(self, **kwargs) -> PreTrainedTokenizer:
+        return self.tokenizer_class.from_pretrained(self.tmpdirname, **kwargs)
 
     def get_rust_tokenizer(self, **kwargs):
         raise NotImplementedError
 
-    def get_input_output_texts(self):
-        raise NotImplementedError
+    def get_input_output_texts(self) -> Tuple[str, str]:
+        """Feel free to overwrite"""
+        # TODO: @property
+        return (
+            "This is a test",
+            "This is a test",
+        )
 
     @staticmethod
     def convert_batch_encode_plus_format_to_encode_plus(batch_encode_plus_sequences):
         # Switch from batch_encode_plus format:   {'input_ids': [[...], [...]], ...}
-        # to the concatenated encode_plus format: [{'input_ids': [...], ...}, {'input_ids': [...], ...}]
+        # to the list of examples/ encode_plus format: [{'input_ids': [...], ...}, {'input_ids': [...], ...}]
         return [
             {value: batch_encode_plus_sequences[value][i] for value in batch_encode_plus_sequences.keys()}
             for i in range(len(batch_encode_plus_sequences["input_ids"]))
@@ -114,13 +119,13 @@ class TokenizerTesterMixin:
 
         # Now let's start the test
         tokenizer = self.get_tokenizer(max_len=42)
-
-        before_tokens = tokenizer.encode("He is very happy, UNwant\u00E9d,running", add_special_tokens=False)
+        sample_text = "He is very happy, UNwant\u00E9d,running"
+        before_tokens = tokenizer.encode(sample_text, add_special_tokens=False)
 
         tokenizer.save_pretrained(self.tmpdirname)
         tokenizer = self.tokenizer_class.from_pretrained(self.tmpdirname)
 
-        after_tokens = tokenizer.encode("He is very happy, UNwant\u00E9d,running", add_special_tokens=False)
+        after_tokens = tokenizer.encode(sample_text, add_special_tokens=False)
         self.assertListEqual(before_tokens, after_tokens)
 
         self.assertEqual(tokenizer.max_len, 42)
@@ -128,6 +133,7 @@ class TokenizerTesterMixin:
         self.assertEqual(tokenizer.max_len, 43)
 
     def test_pickle_tokenizer(self):
+        """Google pickle __getstate__ __setstate__ if you are struggling with this."""
         tokenizer = self.get_tokenizer()
         self.assertIsNotNone(tokenizer)
 
@@ -253,7 +259,7 @@ class TokenizerTesterMixin:
         decoded = tokenizer.decode(encoded, skip_special_tokens=True)
         assert special_token not in decoded
 
-    def test_required_methods_tokenizer(self):
+    def test_internal_consistency(self):
         tokenizer = self.get_tokenizer()
         input_text, output_text = self.get_input_output_texts()
 
@@ -263,13 +269,12 @@ class TokenizerTesterMixin:
         self.assertListEqual(ids, ids_2)
 
         tokens_2 = tokenizer.convert_ids_to_tokens(ids)
+        self.assertNotEqual(len(tokens_2), 0)
         text_2 = tokenizer.decode(ids)
+        self.assertIsInstance(text_2, str)
 
         self.assertEqual(text_2, output_text)
 
-        self.assertNotEqual(len(tokens_2), 0)
-        self.assertIsInstance(text_2, str)
-
     def test_encode_decode_with_spaces(self):
         tokenizer = self.get_tokenizer()
 
@@ -429,10 +434,7 @@ class TokenizerTesterMixin:
 
     def test_special_tokens_mask(self):
         tokenizer = self.get_tokenizer()
-
         sequence_0 = "Encode this."
-        sequence_1 = "This one too please."
-
         # Testing single inputs
         encoded_sequence = tokenizer.encode(sequence_0, add_special_tokens=False)
         encoded_sequence_dict = tokenizer.encode_plus(
@@ -442,13 +444,13 @@ class TokenizerTesterMixin:
         special_tokens_mask = encoded_sequence_dict["special_tokens_mask"]
         self.assertEqual(len(special_tokens_mask), len(encoded_sequence_w_special))
 
-        filtered_sequence = [
-            (x if not special_tokens_mask[i] else None) for i, x in enumerate(encoded_sequence_w_special)
-        ]
-        filtered_sequence = [x for x in filtered_sequence if x is not None]
+        filtered_sequence = [x for i, x in enumerate(encoded_sequence_w_special) if not special_tokens_mask[i]]
         self.assertEqual(encoded_sequence, filtered_sequence)
 
-        # Testing inputs pairs
+    def test_special_tokens_mask_input_pairs(self):
+        tokenizer = self.get_tokenizer()
+        sequence_0 = "Encode this."
+        sequence_1 = "This one too please."
         encoded_sequence = tokenizer.encode(sequence_0, add_special_tokens=False)
         encoded_sequence += tokenizer.encode(sequence_1, add_special_tokens=False)
         encoded_sequence_dict = tokenizer.encode_plus(
@@ -464,7 +466,9 @@ class TokenizerTesterMixin:
         filtered_sequence = [x for x in filtered_sequence if x is not None]
         self.assertEqual(encoded_sequence, filtered_sequence)
 
-        # Testing with already existing special tokens
+    def test_special_tokens_mask_already_has_special_tokens(self):
+        tokenizer = self.get_tokenizer()
+        sequence_0 = "Encode this."
         if tokenizer.cls_token_id == tokenizer.unk_token_id and tokenizer.cls_token_id == tokenizer.unk_token_id:
             tokenizer.add_special_tokens({"cls_token": "</s>", "sep_token": "<s>"})
         encoded_sequence_dict = tokenizer.encode_plus(
@@ -514,13 +518,12 @@ class TokenizerTesterMixin:
         tokenizer.padding_side = "right"
         padded_sequence_right = tokenizer.encode(sequence, pad_to_max_length=True)
         padded_sequence_right_length = len(padded_sequence_right)
+        assert sequence_length == padded_sequence_right_length
+        assert encoded_sequence == padded_sequence_right
 
         tokenizer.padding_side = "left"
         padded_sequence_left = tokenizer.encode(sequence, pad_to_max_length=True)
         padded_sequence_left_length = len(padded_sequence_left)
-
-        assert sequence_length == padded_sequence_right_length
-        assert encoded_sequence == padded_sequence_right
         assert sequence_length == padded_sequence_left_length
         assert encoded_sequence == padded_sequence_left
 
@@ -617,6 +620,9 @@ class TokenizerTesterMixin:
         self.assertIsInstance(vocab, dict)
         self.assertEqual(len(vocab), len(tokenizer))
 
+    def test_conversion_reversible(self):
+        tokenizer = self.get_tokenizer()
+        vocab = tokenizer.get_vocab()
         for word, ind in vocab.items():
             self.assertEqual(tokenizer.convert_tokens_to_ids(word), ind)
             self.assertEqual(tokenizer.convert_ids_to_tokens(ind), word)
@@ -746,6 +752,7 @@ class TokenizerTesterMixin:
 
     @require_torch
     def test_torch_encode_plus_sent_to_model(self):
+        import torch
         from transformers import MODEL_MAPPING, TOKENIZER_MAPPING
 
         MODEL_TOKENIZER_MAPPING = merge_model_tokenizer_mappings(MODEL_MAPPING, TOKENIZER_MAPPING)
@@ -773,8 +780,10 @@ class TokenizerTesterMixin:
         encoded_sequence = tokenizer.encode_plus(sequence, return_tensors="pt")
         batch_encoded_sequence = tokenizer.batch_encode_plus([sequence, sequence], return_tensors="pt")
         # This should not fail
-        model(**encoded_sequence)
-        model(**batch_encoded_sequence)
+
+        with torch.no_grad():  # saves some time
+            model(**encoded_sequence)
+            model(**batch_encoded_sequence)
 
         if self.test_rust_tokenizer:
             fast_tokenizer = self.get_rust_tokenizer()
diff --git a/tests/test_tokenization_distilbert.py b/tests/test_tokenization_distilbert.py
index ac2e447fb3..3a2d12db95 100644
--- a/tests/test_tokenization_distilbert.py
+++ b/tests/test_tokenization_distilbert.py
@@ -24,9 +24,6 @@ class DistilBertTokenizationTest(BertTokenizationTest):
 
     tokenizer_class = DistilBertTokenizer
 
-    def get_tokenizer(self, **kwargs):
-        return DistilBertTokenizer.from_pretrained(self.tmpdirname, **kwargs)
-
     def get_rust_tokenizer(self, **kwargs):
         return DistilBertTokenizerFast.from_pretrained(self.tmpdirname, **kwargs)
 
diff --git a/tests/test_tokenization_openai.py b/tests/test_tokenization_openai.py
index f89ec61ff6..777b80bc1e 100644
--- a/tests/test_tokenization_openai.py
+++ b/tests/test_tokenization_openai.py
@@ -64,13 +64,8 @@ class OpenAIGPTTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
         with open(self.merges_file, "w") as fp:
             fp.write("\n".join(merges))
 
-    def get_tokenizer(self, **kwargs):
-        return OpenAIGPTTokenizer.from_pretrained(self.tmpdirname, **kwargs)
-
     def get_input_output_texts(self):
-        input_text = "lower newer"
-        output_text = "lower newer"
-        return input_text, output_text
+        return "lower newer", "lower newer"
 
     def test_full_tokenizer(self):
         tokenizer = OpenAIGPTTokenizer(self.vocab_file, self.merges_file)
diff --git a/tests/test_tokenization_t5.py b/tests/test_tokenization_t5.py
index 793d80ac64..bee735921c 100644
--- a/tests/test_tokenization_t5.py
+++ b/tests/test_tokenization_t5.py
@@ -37,14 +37,6 @@ class T5TokenizationTest(TokenizerTesterMixin, unittest.TestCase):
         tokenizer = T5Tokenizer(SAMPLE_VOCAB)
         tokenizer.save_pretrained(self.tmpdirname)
 
-    def get_tokenizer(self, **kwargs):
-        return T5Tokenizer.from_pretrained(self.tmpdirname, **kwargs)
-
-    def get_input_output_texts(self):
-        input_text = "This is a test"
-        output_text = "This is a test"
-        return input_text, output_text
-
     def test_full_tokenizer(self):
         tokenizer = T5Tokenizer(SAMPLE_VOCAB)
 
diff --git a/tests/test_tokenization_xlm.py b/tests/test_tokenization_xlm.py
index 4312355427..0b9ca93526 100644
--- a/tests/test_tokenization_xlm.py
+++ b/tests/test_tokenization_xlm.py
@@ -65,9 +65,6 @@ class XLMTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
         with open(self.merges_file, "w") as fp:
             fp.write("\n".join(merges))
 
-    def get_tokenizer(self, **kwargs):
-        return XLMTokenizer.from_pretrained(self.tmpdirname, **kwargs)
-
     def get_input_output_texts(self):
         input_text = "lower newer"
         output_text = "lower newer"
diff --git a/tests/test_tokenization_xlm_roberta.py b/tests/test_tokenization_xlm_roberta.py
index e2433fc7da..57212571e6 100644
--- a/tests/test_tokenization_xlm_roberta.py
+++ b/tests/test_tokenization_xlm_roberta.py
@@ -17,6 +17,7 @@
 import os
 import unittest
 
+from transformers.file_utils import cached_property
 from transformers.tokenization_xlm_roberta import SPIECE_UNDERLINE, XLMRobertaTokenizer
 
 from .test_tokenization_common import TokenizerTesterMixin
@@ -37,14 +38,6 @@ class XLMRobertaTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
         tokenizer = XLMRobertaTokenizer(SAMPLE_VOCAB, keep_accents=True)
         tokenizer.save_pretrained(self.tmpdirname)
 
-    def get_tokenizer(self, **kwargs):
-        return XLMRobertaTokenizer.from_pretrained(self.tmpdirname, **kwargs)
-
-    def get_input_output_texts(self):
-        input_text = "This is a test"
-        output_text = "This is a test"
-        return input_text, output_text
-
     def test_full_tokenizer(self):
         tokenizer = XLMRobertaTokenizer(SAMPLE_VOCAB, keep_accents=True)
 
@@ -121,22 +114,22 @@ class XLMRobertaTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
             ],
         )
 
+    @cached_property
+    def big_tokenizer(self):
+        return XLMRobertaTokenizer.from_pretrained("xlm-roberta-base")
+
     @slow
     def test_tokenization_base_easy_symbols(self):
-        tokenizer = XLMRobertaTokenizer.from_pretrained("xlm-roberta-base")
-
         symbols = "Hello World!"
         original_tokenizer_encodings = [0, 35378, 6661, 38, 2]
         # xlmr = torch.hub.load('pytorch/fairseq', 'xlmr.base')  # xlmr.large has same tokenizer
         # xlmr.eval()
         # xlmr.encode(symbols)
 
-        self.assertListEqual(original_tokenizer_encodings, tokenizer.encode(symbols))
+        self.assertListEqual(original_tokenizer_encodings, self.big_tokenizer.encode(symbols))
 
     @slow
     def test_tokenization_base_hard_symbols(self):
-        tokenizer = XLMRobertaTokenizer.from_pretrained("xlm-roberta-base")
-
         symbols = 'This is a very long text with a lot of weird characters, such as: . , ~ ? ( ) " [ ] ! : - . Also we will add words that should not exsist and be tokenized to <unk>, such as saoneuhaoesuth'
         original_tokenizer_encodings = [
             0,
@@ -209,4 +202,4 @@ class XLMRobertaTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
         # xlmr.eval()
         # xlmr.encode(symbols)
 
-        self.assertListEqual(original_tokenizer_encodings, tokenizer.encode(symbols))
+        self.assertListEqual(original_tokenizer_encodings, self.big_tokenizer.encode(symbols))
diff --git a/tests/test_tokenization_xlnet.py b/tests/test_tokenization_xlnet.py
index 2fa94bfbc9..a56e236a1b 100644
--- a/tests/test_tokenization_xlnet.py
+++ b/tests/test_tokenization_xlnet.py
@@ -37,14 +37,6 @@ class XLNetTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
         tokenizer = XLNetTokenizer(SAMPLE_VOCAB, keep_accents=True)
         tokenizer.save_pretrained(self.tmpdirname)
 
-    def get_tokenizer(self, **kwargs):
-        return XLNetTokenizer.from_pretrained(self.tmpdirname, **kwargs)
-
-    def get_input_output_texts(self):
-        input_text = "This is a test"
-        output_text = "This is a test"
-        return input_text, output_text
-
     def test_full_tokenizer(self):
         tokenizer = XLNetTokenizer(SAMPLE_VOCAB, keep_accents=True)