labels: Union[List[str], List[int]],
    label_namespace: str = 'labels',
) -> None

A SequenceLabelField assigns a categorical label to each element in a Because it's a labeling of some other field, we take that field as input here, and we use it to determine our padding and other things.

This field will get converted into a list of integer class ids, representing the correct class for each element in the sequence.


  • labels : Union[List[str], List[int]] A sequence of categorical labels, encoded as strings or integers. These could be POS tags like [NN, JJ, ...], BIO tags like [B-PERS, I-PERS, O, O, ...], or any other categorical tag sequence. If the labels are encoded as integers, they will not be indexed using a vocab.
  • sequence_field : SequenceField A field containing the sequence that this SequenceLabelField is labeling. Most often, this is a TextField, for tagging individual tokens in a sentence.
  • label_namespace : str, optional (default='labels') The namespace to use for converting tag strings into integers. We convert tag strings to integers for you, and this parameter tells the Vocabulary object which mapping from strings to integers to use (so that "O" as a tag doesn't get the same id as "O" as a word).


    padding_lengths: Dict[str, int],
) -> torch.Tensor

Given a set of specified padding lengths, actually pad the data in this field and return a torch Tensor (or a more complex data structure) of the correct shape. We also take a couple of parameters that are important when constructing torch Tensors.


  • padding_lengths : Dict[str, int] This dictionary will have the same keys that were produced in
  • :func:get_padding_lengths. The values specify the lengths to use when padding each relevant dimension, aggregated across all instances in a batch.


SequenceLabelField.count_vocab_items(self, counter:Dict[str, Dict[str, int]])

If there are strings in this field that need to be converted into integers through a :class:Vocabulary, here is where we count them, to determine which tokens are in or out of the vocabulary.

If your Field does not have any strings that need to be converted into indices, you do not need to implement this method.

A note on this counter: because Fields can represent conceptually different things, we separate the vocabulary items by namespaces. This way, we can use a single shared mechanism to handle all mappings from strings to integers in all fields, while keeping words in a TextField from sharing the same ids with labels in a LabelField (e.g., "entailment" or "contradiction" are labels in an entailment task)

Additionally, a single Field might want to use multiple namespaces - TextFields can be represented as a combination of word ids and character ids, and you don't want words and characters to share the same vocabulary - "a" as a word should get a different id from "a" as a character, and the vocabulary sizes of words and characters are very different.

Because of this, the first key in the counter object is a namespace, like "tokens", "token_characters", "tags", or "labels", and the second key is the actual vocabulary item.


SequenceLabelField.empty_field(self) -> 'SequenceLabelField'

So that ListField can pad the number of fields in a list (e.g., the number of answer option TextFields), we need a representation of an empty field of each type. This returns that. This will only ever be called when we're to the point of calling :func:as_tensor, so you don't need to worry about get_padding_lengths, count_vocab_items, etc., being called on this empty field.

We make this an instance method instead of a static method so that if there is any state in the Field, we can copy it over (e.g., the token indexers in TextField).


SequenceLabelField.get_padding_lengths(self) -> Dict[str, int]

If there are things in this field that need padding, note them here. In order to pad a batch of instance, we get all of the lengths from the batch, take the max, and pad everything to that length (or use a pre-specified maximum length). The return value is a dictionary mapping keys to lengths, like {'num_tokens': 13}.

This is always called after :func:index.



Given a :class:Vocabulary, converts all strings in this field into (typically) integers. This modifies the Field object, it does not return anything.

If your Field does not have any strings that need to be converted into indices, you do not need to implement this method.