CopyNetSeq2Seq(self,, source_embedder:allennlp.modules.text_field_embedders.text_field_embedder.TextFieldEmbedder, encoder:allennlp.modules.seq2seq_encoders.seq2seq_encoder.Seq2SeqEncoder, attention:allennlp.modules.attention.attention.Attention, beam_size:int, max_decoding_steps:int, target_embedding_dim:int=30, copy_token:str='@COPY@', source_namespace:str='source_tokens', target_namespace:str='target_tokens',,, initializer:allennlp.nn.initializers.InitializerApplicator=<allennlp.nn.initializers.InitializerApplicator object at 0x7f34548684a8>) -> None

This is an implementation of CopyNet. CopyNet is a sequence-to-sequence encoder-decoder model with a copying mechanism that can copy tokens from the source sentence into the target sentence instead of generating all target tokens only from the target vocabulary.

It is very similar to a typical seq2seq model used in neural machine translation tasks, for example, except that in addition to providing a "generation" score at each timestep for the tokens in the target vocabulary, it also provides a "copy" score for each token that appears in the source sentence. In other words, you can think of CopyNet as a seq2seq model with a dynamic target vocabulary that changes based on the tokens in the source sentence, allowing it to predict tokens that are out-of-vocabulary (OOV) with respect to the actual target vocab.


  • vocab : Vocabulary, required Vocabulary containing source and target vocabularies.
  • source_embedder : TextFieldEmbedder, required Embedder for source side sequences
  • encoder : Seq2SeqEncoder, required The encoder of the "encoder/decoder" model
  • attention : Attention, required This is used to get a dynamic summary of encoder outputs at each timestep when producing the "generation" scores for the target vocab.
  • beam_size : int, required Beam width to use for beam search prediction.
  • max_decoding_steps : int, required Maximum sequence length of target predictions.
  • target_embedding_dim : int, optional (default = 30) The size of the embeddings for the target vocabulary.
  • copy_token : str, optional (default = '@COPY@') The token used to indicate that a target token was copied from the source. If this token is not already in your target vocabulary, it will be added.
  • source_namespace : str, optional (default = 'source_tokens') The namespace for the source vocabulary.
  • target_namespace : str, optional (default = 'target_tokens') The namespace for the target vocabulary.
  • tensor_based_metric : Metric, optional (default = BLEU) A metric to track on validation data that takes raw tensors when its called.
  • This metric must accept two arguments when called: a batched tensor of predicted token indices, and a batched tensor of gold token indices.
  • token_based_metric : Metric, optional (default = None) A metric to track on validation data that takes lists of lists of tokens as input. This metric must accept two arguments when called, both of type List[List[str]]. The first is a predicted sequence for each item in the batch and the second is a gold sequence for each item in the batch.
  • initializer : InitializerApplicator, optional An initialization strategy for the model weights.


CopyNetSeq2Seq.forward(self, source_tokens:Dict[str, Dict[str, torch.Tensor]], source_token_ids:torch.Tensor, source_to_target:torch.Tensor, metadata:List[Dict[str, Any]], target_tokens:Dict[str, Dict[str, torch.Tensor]]=None, target_token_ids:torch.Tensor=None) -> Dict[str, torch.Tensor]

Make foward pass with decoder logic for producing the entire target sequence.


  • source_tokens : TextFieldTensors, required The output of TextField.as_array() applied on the source TextField. This will be passed through a TextFieldEmbedder and then through an encoder.
  • source_token_ids : torch.Tensor, required Tensor containing IDs that indicate which source tokens match each other.
  • Has shape: (batch_size, trimmed_source_length).
  • source_to_target : torch.Tensor, required Tensor containing vocab index of each source token with respect to the
  • target vocab namespace. Shape: (batch_size, trimmed_source_length).
  • metadata : List[Dict[str, Any]], required Metadata field that contains the original source tokens with key 'source_tokens' and any other meta fields. When 'target_tokens' is also passed, the metadata should also contain the original target tokens with key 'target_tokens'.
  • target_tokens : TextFieldTensors, optional (default = None) Output of Textfield.as_array() applied on target TextField. We assume that the target tokens are also represented as a TextField which must contain a "tokens" key that uses single ids.
  • target_token_ids : torch.Tensor, optional (default = None) A tensor of shape (batch_size, target_sequence_length) which indicates which tokens in the target sequence match tokens in the source sequence.


Dict[str, torch.Tensor]


CopyNetSeq2Seq.take_search_step(self, last_predictions:torch.Tensor, state:Dict[str, torch.Tensor]) -> Tuple[torch.Tensor, Dict[str, torch.Tensor]]

Take step during beam search.

This function is what gets passed to the method. It takes predictions from the last timestep and the current state and outputs the log probabilities assigned to tokens for the next timestep, as well as the updated state.

Since we are predicting tokens out of the extended vocab (target vocab + all unique tokens from the source sentence), this is a little more complicated that just making a forward pass through the model. The output log probs will have shape (group_size, target_vocab_size + trimmed_source_length) so that each token in the target vocab and source sentence are assigned a probability.

Note that copy scores are assigned to each source token based on their position, not unique value. So if a token appears more than once in the source sentence, it will have more than one score. Further, if a source token is also part of the target vocab, its final score will be the sum of the generation and copy scores. Therefore, in order to get the score for all tokens in the extended vocab at this step, we have to combine copy scores for re-occuring source tokens and potentially add them to the generation scores for the matching token in the target vocab, if there is one.

So we can break down the final log probs output as the concatenation of two matrices, A: (group_size, target_vocab_size), and B: (group_size, trimmed_source_length). Matrix A contains the sum of the generation score and copy scores (possibly 0) for each target token. Matrix B contains left-over copy scores for source tokens that do NOT appear in the target vocab, with zeros everywhere else. But since a source token may appear more than once in the source sentence, we also have to sum the scores for each appearance of each unique source token. So matrix B actually only has non-zero values at the first occurence of each source token that is not in the target vocab.


  • last_predictions : torch.Tensor
  • Shape: (group_size,)

  • state : Dict[str, torch.Tensor] Contains all state tensors necessary to produce generation and copy scores for next step.


group_size != batch_size. In fact, group_size = batch_size * beam_size.


CopyNetSeq2Seq.decode(self, output_dict:Dict[str, torch.Tensor]) -> Dict[str, Any]

Finalize predictions.

After a beam search, the predicted indices correspond to tokens in the target vocabulary OR tokens in source sentence. Here we gather the actual tokens corresponding to the indices.


CopyNetSeq2Seq.get_metrics(self, reset:bool=False) -> Dict[str, float]

Returns a dictionary of metrics. This method will be called by in order to compute and use model metrics for early stopping and model serialization. We return an empty dictionary here rather than raising as it is not required to implement metrics for a new model. A boolean reset parameter is passed, as frequently a metric accumulator will have some state which should be reset between epochs. This is also compatible with Metrics should be populated during the call to forward, with the handling the accumulation of the metric until this method is called.