Always truncate argument in the encode method

transformers/tests/tokenization_tests_commons.py

@@ -232,6 +232,23 @@ class CommonTestCases:
             assert len(truncated_sequence) == total_length - 2
             assert truncated_sequence == tokenizer.add_special_tokens_single_sequence(sequence[:-2])
 
+        def test_always_truncate(self):
+            tokenizer = self.get_tokenizer()
+
+            seq_0 = "This is a sentence to be encoded."
+            length_single_sequence = len(tokenizer.encode(seq_0))
+            length = len(tokenizer.encode(seq_0, seq_0, add_special_tokens=True))
+
+            not_truncated = tokenizer.encode(seq_0, seq_0, add_special_tokens=True, max_length=length_single_sequence)
+            truncated = tokenizer.encode(
+                seq_0, seq_0,
+                max_length=length_single_sequence,
+                add_special_tokens=True,
+                always_truncate=True
+            )
+
+            assert truncated == not_truncated[:length_single_sequence - length]
+
         def test_maximum_encoding_length_pair_input(self):
             tokenizer = self.get_tokenizer()