MaskedLanguageModel(self,, text_field_embedder:allennlp.modules.text_field_embedders.text_field_embedder.TextFieldEmbedder, language_model_head:allennlp.modules.language_model_heads.language_model_head.LanguageModelHead, contextualizer:allennlp.modules.seq2seq_encoders.seq2seq_encoder.Seq2SeqEncoder=None, target_namespace:str='bert', dropout:float=0.0, initializer:allennlp.nn.initializers.InitializerApplicator=None) -> None

The MaskedLanguageModel embeds some input tokens (including some which are masked), contextualizes them, then predicts targets for the masked tokens, computing a loss against known targets.

NOTE: This was developed for use in a demo, not for training. It's possible that it will still work for training a masked LM, but it is very likely that some other code would be much more efficient for that. This does compute correct gradients of the loss, because we use that in our demo, so in principle it should be able to train a model, we just don't necessarily endorse that use.


  • vocab : Vocabulary
  • text_field_embedder : TextFieldEmbedder Used to embed the indexed tokens we get in forward.
  • language_model_head : LanguageModelHead The torch.nn.Module that goes from the hidden states output by the contextualizer to logits over some output vocabulary.
  • contextualizer : Seq2SeqEncoder, optional (default=None) Used to "contextualize" the embeddings. This is optional because the contextualization might actually be done in the text field embedder.
  • target_namespace : str, optional (default='bert') Namespace to use to convert predicted token ids to strings in Model.decode.
  • dropout : float, optional (default=0.0) If specified, dropout is applied to the contextualized embeddings before computation of the softmax. The contextualized embeddings themselves are returned without dropout.


MaskedLanguageModel.forward(self, tokens:Dict[str, Dict[str, torch.Tensor]], mask_positions:torch.LongTensor, target_ids:Dict[str, Dict[str, torch.Tensor]]=None) -> Dict[str, torch.Tensor]


  • tokens : TextFieldTensors The output of TextField.as_tensor() for a batch of sentences.
  • mask_positions : torch.LongTensor The positions in tokens that correspond to [MASK] tokens that we should try to fill in. Shape should be (batch_size, num_masks).
  • target_ids : TextFieldTensors This is a list of token ids that correspond to the mask positions we're trying to fill. It is the output of a TextField, purely for convenience, so we can handle wordpiece tokenizers and such without having to do crazy things in the dataset reader. We assume that there is exactly one entry in the dictionary, and that it has a shape identical to mask_positions - one target token per mask position.


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

Takes the result of :func:forward and runs inference / decoding / whatever post-processing you need to do your model. The intent is that model.forward() should produce potentials or probabilities, and then model.decode() can take those results and run some kind of beam search or constrained inference or whatever is necessary. This does not handle all possible decoding use cases, but it at least handles simple kinds of decoding.

This method modifies the input dictionary, and also returns the same dictionary.

By default in the base class we do nothing. If your model has some special decoding step, override this method.