Special tokens should be properly encoded by text_encoders

[ricardorei]

Expected Behavior

encoder = MosesEncoder( ["<s> hello This ain't funny. </s>", "<s> Don't? </s>"])
print (encoder.encode("<s> hello </s>"))

--CONSOLE---
tensor([3, 5, 2])

Actual Behavior

encoder = MosesEncoder( ["<s> hello This ain't funny. </s>", "<s> Don't? </s>"])
print (encoder.encode("<s> hello </s>"))

--CONSOLE---
tensor([ 5, 6, 7, 8, 5, 14, 6, 7])

Explanation

Most if this tokenizers are not aware of this special tokens and end up splitting the special token into different tokens. For instance the '<s>' token becames '<', 's', '>'.

My solution to this problem was to create a method for masking special tokens and another one to restore them in place.

   def _mask_reserved_tokens(self, sequence):
        reserved_tokens = re.findall(r'\<pad\>|\<unk\>|\</s\>|\<s\>|\<copy\>', sequence)
        sequence = re.sub(r'\<pad\>|\<unk\>|\</s\>|\<s\>|\<copy\>', "RESERVEDTOKENMASK", sequence)
        return reserved_tokens, sequence

    def _restore_reserved_tokens(self, reserved_tokens, sequence):
        sequence = _detokenize(sequence)
        for token in reserved_tokens:
            sequence = sequence.replace('RESERVEDTOKENMASK', token, 1)
        return _tokenize(sequence)

Then the encode function becames:

def encode(self, sequence):
        """ Encodes a ``sequence``.
        Args:
            sequence (str): String ``sequence`` to encode.
        Returns:
            torch.Tensor: Encoding of the ``sequence``.
        """
        sequence = super().encode(sequence)
        reserved_tokens, sequence = self._mask_reserved_tokens(sequence)
        sequence = self.tokenize(sequence)
        sequence = self._restore_reserved_tokens(reserved_tokens, sequence)
        vector = [self.stoi.get(token, self.unknown_index) for token in sequence]
        if self.append_eos:
            vector.append(self.eos_index)
        return torch.tensor(vector)

I dont know if this is just a problem that I have but If not I believe that this should be handled natively.

[PetrochukM]

Thanks for submitting this issue.

Is there a particular reason that the end of sentence token and the beginning of sentence token cannot be added after the fact?

---

By the way, your solution may be the correct approach, Google's Subword Tokenizer handles special tokens in a similar way: https://github.com/PetrochukM/PyTorch-NLP/blob/master/torchnlp/encoders/text/subword_text_tokenizer.py#L101

They use unicode to encode various special characters.

[ricardorei]

In my case im building sequence to sequence models and the decoder needs the bos and eos tokens.

Lets supose you have the following sample:

[{'source': 'hello world!', 'target': 'hello'}, {'source': 'Bye', 'target': 'goodbye'}]

If you apply collate vector:

{'source': ['hello world!', 'Bye' ], 'target': ['hello', 'goodbye'] }

now if you apply the batch_encode dor the 'source' you get something like:

tensors = torch.tensor([[7, 19],[14, 0]])
lengths = torch.tensor([2,1])

If now I want to add the BOS and EOS token indexes its harder because I have to expand the tensors and increment the lengths... I just think its easier to add this tokens directly into the strings before vectorization.

[PetrochukM]

Great. Sure thing.

There is an append_eos parameter that helps with appending an EOS token. I'd be happy to accept a PR for append_sos or append_bos parameter to be included.

[ksopyla]

I have experienced similar problem when using CharacterEncoder

tokenizer.encode('<s>hello</s>')

gives

tensor([ 1,  8,  1, 39, 14,7,  7,11,  1,  1,  8,  1])

where token with id=1 is <unk>
I prefere adding eos and sos (bos) tokens by tokenizer, it simplifies all the processing in seq2seq scenario

[PetrochukM]

Thanks for your contribution @wxl1999! append_sos is now supported :)