span_start: int,
    span_end: int,
) -> None

A SpanField is a pair of inclusive, zero-indexed (start, end) indices into a, used to represent a span of text. Because it's a pair of indices into a :class:SequenceField, we take one of those as input to make the span's dependence explicit and to validate that the span is well defined.


  • span_start : int, required.
  • The index of the start of the span in the :class:SequenceField.
  • span_end : int, required.
  • The inclusive index of the end of the span in the :class:SequenceField.
  • sequence_field : SequenceField, required. A field containing the sequence that this SpanField is a span inside.


SpanField.as_tensor(self, 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.



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).


SpanField.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.