Assorted utilities for working with neural networks in AllenNLP.

allennlp.nn.util.add_positional_features(tensor: torch.Tensor, min_timescale: float = 1.0, max_timescale: float = 10000.0)[source]

Implements the frequency-based positional encoding described in Attention is all you Need .

Adds sinusoids of different frequencies to a Tensor. A sinusoid of a different frequency and phase is added to each dimension of the input Tensor. This allows the attention heads to use absolute and relative positions.

The number of timescales is equal to hidden_dim / 2 within the range (min_timescale, max_timescale). For each timescale, the two sinusoidal signals sin(timestep / timescale) and cos(timestep / timescale) are generated and concatenated along the hidden_dim dimension.


a Tensor with shape (batch_size, timesteps, hidden_dim).

min_timescalefloat, optional (default = 1.0)

The smallest timescale to use.

max_timescalefloat, optional (default = 1.0e4)

The largest timescale to use.

The input tensor augmented with the sinusoidal frequencies.
allennlp.nn.util.add_sentence_boundary_token_ids(tensor: torch.Tensor, mask: torch.Tensor, sentence_begin_token: Any, sentence_end_token: Any) → Tuple[torch.Tensor, torch.Tensor][source]

Add begin/end of sentence tokens to the batch of sentences. Given a batch of sentences with size (batch_size, timesteps) or (batch_size, timesteps, dim) this returns a tensor of shape (batch_size, timesteps + 2) or (batch_size, timesteps + 2, dim) respectively.

Returns both the new tensor and updated mask.


A tensor of shape (batch_size, timesteps) or (batch_size, timesteps, dim)


A tensor of shape (batch_size, timesteps)

sentence_begin_token: Any (anything that can be broadcast in torch for assignment)

For 2D input, a scalar with the <S> id. For 3D input, a tensor with length dim.

sentence_end_token: Any (anything that can be broadcast in torch for assignment)

For 2D input, a scalar with the </S> id. For 3D input, a tensor with length dim.


The tensor with the appended and prepended boundary tokens. If the input was 2D, it has shape (batch_size, timesteps + 2) and if the input was 3D, it has shape (batch_size, timesteps + 2, dim).


The new mask for the tensor, taking into account the appended tokens marking the beginning and end of the sentence.

allennlp.nn.util.batch_tensor_dicts(tensor_dicts: List[Dict[str, torch.Tensor]], remove_trailing_dimension: bool = False) → Dict[str, torch.Tensor][source]

Takes a list of tensor dictionaries, where each dictionary is assumed to have matching keys, and returns a single dictionary with all tensors with the same key batched together.

tensor_dictsList[Dict[str, torch.Tensor]]

The list of tensor dictionaries to batch.


If True, we will check for a trailing dimension of size 1 on the tensors that are being batched, and remove it if we find it.

allennlp.nn.util.batched_index_select(target: torch.Tensor, indices: torch.LongTensor, flattened_indices: Union[torch.LongTensor, NoneType] = None) → torch.Tensor[source]

The given indices of size (batch_size, d_1, ..., d_n) indexes into the sequence dimension (dimension 2) of the target, which has size (batch_size, sequence_length, embedding_size).

This function returns selected values in the target with respect to the provided indices, which have size (batch_size, d_1, ..., d_n, embedding_size). This can use the optionally precomputed flattened_indices() with size (batch_size * d_1 * ... * d_n) if given.

An example use case of this function is looking up the start and end indices of spans in a sequence tensor. This is used in the CoreferenceResolver. Model to select contextual word representations corresponding to the start and end indices of mentions. The key reason this can’t be done with basic torch functions is that we want to be able to use look-up tensors with an arbitrary number of dimensions (for example, in the coref model, we don’t know a-priori how many spans we are looking up).

targettorch.Tensor, required.

A 3 dimensional tensor of shape (batch_size, sequence_length, embedding_size). This is the tensor to be indexed.


A tensor of shape (batch_size, …), where each element is an index into the sequence_length dimension of the target tensor.

flattened_indicesOptional[torch.Tensor], optional (default = None)

An optional tensor representing the result of calling :func:~`flatten_and_batch_shift_indices` on indices. This is helpful in the case that the indices can be flattened once and cached for many batch lookups.


A tensor with shape [indices.size(), target.size(-1)] representing the embedded indices extracted from the batch flattened target tensor.

allennlp.nn.util.bucket_values(distances: torch.Tensor, num_identity_buckets: int = 4, num_total_buckets: int = 10) → torch.Tensor[source]

Places the given values (designed for distances) into num_total_buckets``semi-logscale buckets, with ``num_identity_buckets of these capturing single values.

The default settings will bucket values into the following buckets: [0, 1, 2, 3, 4, 5-7, 8-15, 16-31, 32-63, 64+].

distancestorch.Tensor, required.

A Tensor of any size, to be bucketed.

num_identity_buckets: int, optional (default = 4).

The number of identity buckets (those only holding a single value).

num_total_bucketsint, (default = 10)

The total number of buckets to bucket values into.

A tensor of the same shape as the input, containing the indices of the buckets
the values were placed in.
allennlp.nn.util.clamp_tensor(tensor, minimum, maximum)[source]

Supports sparse and dense tensors. Returns a tensor with values clamped between the provided minimum and maximum, without modifying the original tensor.

allennlp.nn.util.clone(module: torch.nn.modules.module.Module, num_copies: int) → torch.nn.modules.container.ModuleList[source]

Produce N identical layers.

allennlp.nn.util.combine_initial_dims(tensor: torch.Tensor) → torch.Tensor[source]

Given a (possibly higher order) tensor of ids with shape (d1, …, dn, sequence_length) Return a view that’s (d1 * … * dn, sequence_length). If original tensor is 1-d or 2-d, return it as is.

allennlp.nn.util.combine_tensors(combination: str, tensors: List[torch.Tensor]) → torch.Tensor[source]

Combines a list of tensors using element-wise operations and concatenation, specified by a combination string. The string refers to (1-indexed) positions in the input tensor list, and looks like "1,2,1+2,3-1".

We allow the following kinds of combinations: x, x*y, x+y, x-y, and x/y, where x and y are positive integers less than or equal to len(tensors). Each of the binary operations is performed elementwise. You can give as many combinations as you want in the combination string. For example, for the input string "1,2,1*2", the result would be [1;2;1*2], as you would expect, where [;] is concatenation along the last dimension.

If you have a fixed, known way to combine tensors that you use in a model, you should probably just use something like[x_tensor, y_tensor, x_tensor * y_tensor]). This function adds some complexity that is only necessary if you want the specific combination used to be configurable.

If you want to do any element-wise operations, the tensors involved in each element-wise operation must have the same shape.

This function also accepts x and y in place of 1 and 2 in the combination string.

allennlp.nn.util.combine_tensors_and_multiply(combination: str, tensors: List[torch.Tensor], weights: torch.nn.parameter.Parameter) → torch.Tensor[source]

Like combine_tensors(), but does a weighted (linear) multiplication while combining. This is a separate function from combine_tensors because we try to avoid instantiating large intermediate tensors during the combination, which is possible because we know that we’re going to be multiplying by a weight vector in the end.


Same as in combine_tensors()


A list of tensors to combine, where the integers in the combination are (1-indexed) positions in this list of tensors. These tensors are all expected to have either three or four dimensions, with the final dimension being an embedding. If there are four dimensions, one of them must have length 1.


A vector of weights to use for the combinations. This should have shape (combined_dim,), as calculated by get_combined_dim().

allennlp.nn.util.device_mapping(cuda_device: int)[source]

In order to torch.load() a GPU-trained model onto a CPU (or specific GPU), you have to supply a map_location function. Call this with the desired cuda_device to get the function that torch.load() needs.

allennlp.nn.util.find_embedding_layer(model: torch.nn.modules.module.Module) → torch.nn.modules.module.Module[source]

Takes a model (typically an AllenNLP Model, but this works for any torch.nn.Module) and makes a best guess about which module is the embedding layer. For typical AllenNLP models, this often is the TextFieldEmbedder, but if you’re using a pre-trained contextualizer, we really want layer 0 of that contextualizer, not the output. So there are a bunch of hacks in here for specific pre-trained contextualizers.

allennlp.nn.util.flatten_and_batch_shift_indices(indices: torch.Tensor, sequence_length: int) → torch.Tensor[source]

This is a subroutine for batched_index_select(). The given indices of size (batch_size, d_1, ..., d_n) indexes into dimension 2 of a target tensor, which has size (batch_size, sequence_length, embedding_size). This function returns a vector that correctly indexes into the flattened target. The sequence length of the target must be provided to compute the appropriate offsets.

indices = torch.ones([2,3], dtype=torch.long)
# Sequence length of the target tensor.
sequence_length = 10
shifted_indices = flatten_and_batch_shift_indices(indices, sequence_length)
# Indices into the second element in the batch are correctly shifted
# to take into account that the target tensor will be flattened before
# the indices are applied.
assert shifted_indices == [1, 1, 1, 11, 11, 11]
indicestorch.LongTensor, required.
sequence_lengthint, required.

The length of the sequence the indices index into. This must be the second dimension of the tensor.

allennlp.nn.util.flattened_index_select(target: torch.Tensor, indices: torch.LongTensor) → torch.Tensor[source]

The given indices of size (set_size, subset_size) specifies subsets of the target that each of the set_size rows should select. The target has size (batch_size, sequence_length, embedding_size), and the resulting selected tensor has size (batch_size, set_size, subset_size, embedding_size).

targettorch.Tensor, required.

A Tensor of shape (batch_size, sequence_length, embedding_size).

indicestorch.LongTensor, required.

A LongTensor of shape (set_size, subset_size). All indices must be < sequence_length as this tensor is an index into the sequence_length dimension of the target.

selectedtorch.Tensor, required.

A Tensor of shape (batch_size, set_size, subset_size, embedding_size).

allennlp.nn.util.get_combined_dim(combination: str, tensor_dims: List[int]) → int[source]

For use with combine_tensors(). This function computes the resultant dimension when calling combine_tensors(combination, tensors), when the tensor dimension is known. This is necessary for knowing the sizes of weight matrices when building models that use combine_tensors.


A comma-separated list of combination pieces, like "1,2,1*2", specified identically to combination in combine_tensors().


A list of tensor dimensions, where each dimension is from the last axis of the tensors that will be input to combine_tensors().

allennlp.nn.util.get_device_of(tensor: torch.Tensor) → int[source]

Returns the device of the tensor.

allennlp.nn.util.get_dropout_mask(dropout_probability: float, tensor_for_masking: torch.Tensor)[source]

Computes and returns an element-wise dropout mask for a given tensor, where each element in the mask is dropped out with probability dropout_probability. Note that the mask is NOT applied to the tensor - the tensor is passed to retain the correct CUDA tensor type for the mask.

dropout_probabilityfloat, required.

Probability of dropping a dimension of the input.

tensor_for_maskingtorch.Tensor, required.
A torch.FloatTensor consisting of the binary mask scaled by 1/ (1 - dropout_probability).
This scaling ensures expected values and variances of the output of applying this mask

and the original tensor are the same.

allennlp.nn.util.get_final_encoder_states(encoder_outputs: torch.Tensor, mask: torch.Tensor, bidirectional: bool = False) → torch.Tensor[source]

Given the output from a Seq2SeqEncoder, with shape (batch_size, sequence_length, encoding_dim), this method returns the final hidden state for each element of the batch, giving a tensor of shape (batch_size, encoding_dim). This is not as simple as encoder_outputs[:, -1], because the sequences could have different lengths. We use the mask (which has shape (batch_size, sequence_length)) to find the final state for each batch instance.

Additionally, if bidirectional is True, we will split the final dimension of the encoder_outputs into two and assume that the first half is for the forward direction of the encoder and the second half is for the backward direction. We will concatenate the last state for each encoder dimension, giving encoder_outputs[:, -1, :encoding_dim/2] concatenated with encoder_outputs[:, 0, encoding_dim/2:].

allennlp.nn.util.get_lengths_from_binary_sequence_mask(mask: torch.Tensor)[source]

Compute sequence lengths for each batch element in a tensor using a binary mask.

masktorch.Tensor, required.

A 2D binary mask of shape (batch_size, sequence_length) to calculate the per-batch sequence lengths from.

A torch.LongTensor of shape (batch_size,) representing the lengths
of the sequences in the batch.
allennlp.nn.util.get_mask_from_sequence_lengths(sequence_lengths: torch.Tensor, max_length: int) → torch.Tensor[source]

Given a variable of shape (batch_size,) that represents the sequence lengths of each batch element, this function returns a (batch_size, max_length) mask variable. For example, if our input was [2, 2, 3], with a max_length of 4, we’d return [[1, 1, 0, 0], [1, 1, 0, 0], [1, 1, 1, 0]].

We require max_length here instead of just computing it from the input sequence_lengths because it lets us avoid finding the max, then copying that value from the GPU to the CPU so that we can use it to construct a new tensor.

allennlp.nn.util.get_range_vector(size: int, device: int) → torch.Tensor[source]

Returns a range vector with the desired size, starting at 0. The CUDA implementation is meant to avoid copy data from CPU to GPU.

allennlp.nn.util.get_text_field_mask(text_field_tensors: Dict[str, torch.Tensor], num_wrapping_dims: int = 0) → torch.LongTensor[source]

Takes the dictionary of tensors produced by a TextField and returns a mask with 0 where the tokens are padding, and 1 otherwise. We also handle TextFields wrapped by an arbitrary number of ListFields, where the number of wrapping ListFields is given by num_wrapping_dims.

If num_wrapping_dims == 0, the returned mask has shape (batch_size, num_tokens). If num_wrapping_dims > 0 then the returned mask has num_wrapping_dims extra dimensions, so the shape will be (batch_size, ..., num_tokens).

There could be several entries in the tensor dictionary with different shapes (e.g., one for word ids, one for character ids). In order to get a token mask, we use the tensor in the dictionary with the lowest number of dimensions. After subtracting num_wrapping_dims, if this tensor has two dimensions we assume it has shape (batch_size, ..., num_tokens), and use it for the mask. If instead it has three dimensions, we assume it has shape (batch_size, ..., num_tokens, num_features), and sum over the last dimension to produce the mask. Most frequently this will be a character id tensor, but it could also be a featurized representation of each token, etc.

If the input text_field_tensors contains the “mask” key, this is returned instead of inferring the mask.

TODO(joelgrus): can we change this? NOTE: Our functions for generating masks create torch.LongTensors, because using torch.ByteTensors makes it easy to run into overflow errors when doing mask manipulation, such as summing to get the lengths of sequences - see below. >>> mask = torch.ones([260]).byte() >>> mask.sum() # equals 260. >>> var_mask = torch.autograd.V(mask) >>> var_mask.sum() # equals 4, due to 8 bit precision - the sum overflows.

allennlp.nn.util.has_tensor(obj) → bool[source]

Given a possibly complex data structure, check if it has any torch.Tensors in it.

allennlp.nn.util.inspect_parameters(module: torch.nn.modules.module.Module, quiet: bool = False) → Dict[str, Any][source]

Inspects the model/module parameters and their tunability. The output is structured in a nested dict so that parameters in same sub-modules are grouped together. This can be helpful to setup module path based regex, for example in initializer. It prints it by default (optional) and returns the inspection dict. Eg. output:

    "_text_field_embedder": {
        "token_embedder_tokens": {
            "_projection": {
                "bias": "tunable",
                "weight": "tunable"
            "weight": "frozen"
allennlp.nn.util.logsumexp(tensor: torch.Tensor, dim: int = -1, keepdim: bool = False) → torch.Tensor[source]

A numerically stable computation of logsumexp. This is mathematically equivalent to tensor.exp().sum(dim, keep=keepdim).log(). This function is typically used for summing log probabilities.

tensortorch.FloatTensor, required.

A tensor of arbitrary size.

dimint, optional (default = -1)

The dimension of the tensor to apply the logsumexp to.

keepdim: bool, optional (default = False)

Whether to retain a dimension of size one at the dimension we reduce over.

allennlp.nn.util.masked_flip(padded_sequence: torch.Tensor, sequence_lengths: List[int]) → torch.Tensor[source]

Flips a padded tensor along the time dimension without affecting masked entries.


The tensor to flip along the time dimension. Assumed to be of dimensions (batch size, num timesteps, …)


A list containing the lengths of each unpadded sequence in the batch.

A torch.Tensor of the same shape as padded_sequence.
allennlp.nn.util.masked_log_softmax(vector: torch.Tensor, mask: torch.Tensor, dim: int = -1) → torch.Tensor[source]

torch.nn.functional.log_softmax(vector) does not work if some elements of vector should be masked. This performs a log_softmax on just the non-masked portions of vector. Passing None in for the mask is also acceptable; you’ll just get a regular log_softmax.

vector can have an arbitrary number of dimensions; the only requirement is that mask is broadcastable to vector's shape. If mask has fewer dimensions than vector, we will unsqueeze on dimension 1 until they match. If you need a different unsqueezing of your mask, do it yourself before passing the mask into this function.

In the case that the input vector is completely masked, the return value of this function is arbitrary, but not nan. You should be masking the result of whatever computation comes out of this in that case, anyway, so the specific values returned shouldn’t matter. Also, the way that we deal with this case relies on having single-precision floats; mixing half-precision floats with fully-masked vectors will likely give you nans.

If your logits are all extremely negative (i.e., the max value in your logit vector is -50 or lower), the way we handle masking here could mess you up. But if you’ve got logit values that extreme, you’ve got bigger problems than this.

allennlp.nn.util.masked_max(vector: torch.Tensor, mask: torch.Tensor, dim: int, keepdim: bool = False, min_val: float = -10000000.0) → torch.Tensor[source]

To calculate max along certain dimensions on masked values


The vector to calculate max, assume unmasked parts are already zeros


The mask of the vector. It must be broadcastable with vector.


The dimension to calculate max


Whether to keep dimension


The minimal value for paddings

A torch.Tensor of including the maximum values.
allennlp.nn.util.masked_mean(vector: torch.Tensor, mask: torch.Tensor, dim: int, keepdim: bool = False, eps: float = 1e-08) → torch.Tensor[source]

To calculate mean along certain dimensions on masked values


The vector to calculate mean.


The mask of the vector. It must be broadcastable with vector.


The dimension to calculate mean


Whether to keep dimension


A small value to avoid zero division problem.

A torch.Tensor of including the mean values.
allennlp.nn.util.masked_softmax(vector: torch.Tensor, mask: torch.Tensor, dim: int = -1, memory_efficient: bool = False, mask_fill_value: float = -1e+32) → torch.Tensor[source]

torch.nn.functional.softmax(vector) does not work if some elements of vector should be masked. This performs a softmax on just the non-masked portions of vector. Passing None in for the mask is also acceptable; you’ll just get a regular softmax.

vector can have an arbitrary number of dimensions; the only requirement is that mask is broadcastable to vector's shape. If mask has fewer dimensions than vector, we will unsqueeze on dimension 1 until they match. If you need a different unsqueezing of your mask, do it yourself before passing the mask into this function.

If memory_efficient is set to true, we will simply use a very large negative number for those masked positions so that the probabilities of those positions would be approximately 0. This is not accurate in math, but works for most cases and consumes less memory.

In the case that the input vector is completely masked and memory_efficient is false, this function returns an array of 0.0. This behavior may cause NaN if this is used as the last layer of a model that uses categorical cross-entropy loss. Instead, if memory_efficient is true, this function will treat every element as equal, and do softmax over equal numbers.

allennlp.nn.util.move_to_device(obj, cuda_device: int)[source]

Given a structure (possibly) containing Tensors on the CPU, move all the Tensors to the specified GPU (or do nothing, if they should be on the CPU).

allennlp.nn.util.remove_sentence_boundaries(tensor: torch.Tensor, mask: torch.Tensor) → Tuple[torch.Tensor, torch.Tensor][source]

Remove begin/end of sentence embeddings from the batch of sentences. Given a batch of sentences with size (batch_size, timesteps, dim) this returns a tensor of shape (batch_size, timesteps - 2, dim) after removing the beginning and end sentence markers. The sentences are assumed to be padded on the right, with the beginning of each sentence assumed to occur at index 0 (i.e., mask[:, 0] is assumed to be 1).

Returns both the new tensor and updated mask.

This function is the inverse of add_sentence_boundary_token_ids.


A tensor of shape (batch_size, timesteps, dim)


A tensor of shape (batch_size, timesteps)


The tensor after removing the boundary tokens of shape (batch_size, timesteps - 2, dim)


The new mask for the tensor of shape (batch_size, timesteps - 2).

allennlp.nn.util.replace_masked_values(tensor: torch.Tensor, mask: torch.Tensor, replace_with: float) → torch.Tensor[source]

Replaces all masked values in tensor with replace_with. mask must be broadcastable to the same shape as tensor. We require that tensor.dim() == mask.dim(), as otherwise we won’t know which dimensions of the mask to unsqueeze.

This just does tensor.masked_fill(), except the pytorch method fills in things with a mask value of 1, where we want the opposite. You can do this in your own code with tensor.masked_fill((1 - mask).to(dtype=torch.bool), replace_with).

allennlp.nn.util.sequence_cross_entropy_with_logits(logits: torch.FloatTensor, targets: torch.LongTensor, weights: torch.FloatTensor, average: str = 'batch', label_smoothing: float = None, gamma: float = None, alpha: Union[float, List[float], torch.FloatTensor] = None) → torch.FloatTensor[source]

Computes the cross entropy loss of a sequence, weighted with respect to some user provided weights. Note that the weighting here is not the same as in the torch.nn.CrossEntropyLoss() criterion, which is weighting classes; here we are weighting the loss contribution from particular elements in the sequence. This allows loss computations for models which use padding.

logitstorch.FloatTensor, required.

A torch.FloatTensor of size (batch_size, sequence_length, num_classes) which contains the unnormalized probability for each class.

targetstorch.LongTensor, required.

A torch.LongTensor of size (batch, sequence_length) which contains the index of the true class for each corresponding step.

weightstorch.FloatTensor, required.

A torch.FloatTensor of size (batch, sequence_length)

average: str, optional (default = “batch”)

If “batch”, average the loss across the batches. If “token”, average the loss across each item in the input. If None, return a vector of losses per batch element.

label_smoothingfloat, optional (default = None)

Whether or not to apply label smoothing to the cross-entropy loss. For example, with a label smoothing value of 0.2, a 4 class classification target would look like [0.05, 0.05, 0.85, 0.05] if the 3rd class was the correct label.

gammafloat, optional (default = None)

Focal loss[*] focusing parameter gamma to reduces the relative loss for well-classified examples and put more focus on hard. The greater value gamma is, the more focus on hard examples.

alphafloat or List[float], optional (default = None)

Focal loss[*] weighting factor alpha to balance between classes. Can be used independently with gamma. If a single float is provided, it is assumed binary case using alpha and 1 - alpha for positive and negative respectively. If a list of float is provided, with the same length as the number of classes, the weights will match the classes. [*] T. Lin, P. Goyal, R. Girshick, K. He and P. Dollár, “Focal Loss for Dense Object Detection,” 2017 IEEE International Conference on Computer Vision (ICCV), Venice, 2017, pp. 2999-3007.

A torch.FloatTensor representing the cross entropy loss.
If average=="batch" or average=="token", the returned loss is a scalar.
If average is None, the returned loss is a vector of shape (batch_size,).
allennlp.nn.util.sort_batch_by_length(tensor: torch.Tensor, sequence_lengths: torch.Tensor)[source]

Sort a batch first tensor by some specified lengths.

tensortorch.FloatTensor, required.

A batch first Pytorch tensor.

sequence_lengthstorch.LongTensor, required.

A tensor representing the lengths of some dimension of the tensor which we want to sort by.


The original tensor sorted along the batch dimension with respect to sequence_lengths.


The original sequence_lengths sorted by decreasing size.


Indices into the sorted_tensor such that sorted_tensor.index_select(0, restoration_indices) == original_tensor


The indices used to sort the tensor. This is useful if you want to sort many tensors using the same ordering.

allennlp.nn.util.tensors_equal(tensor1: torch.Tensor, tensor2: torch.Tensor, tolerance: float = 1e-12) → bool[source]

A check for tensor equality (by value). We make sure that the tensors have the same shape, then check all of the entries in the tensor for equality. We additionally allow the input tensors to be lists or dictionaries, where we then do the above check on every position in the list / item in the dictionary. If we find objects that aren’t tensors as we’re doing that, we just defer to their equality check.

This is kind of a catch-all method that’s designed to make implementing __eq__ methods easier, in a way that’s really only intended to be useful for tests.

allennlp.nn.util.uncombine_initial_dims(tensor: torch.Tensor, original_size: torch.Size) → torch.Tensor[source]

Given a tensor of embeddings with shape (d1 * … * dn, sequence_length, embedding_dim) and the original shape (d1, …, dn, sequence_length), return the reshaped tensor of embeddings with shape (d1, …, dn, sequence_length, embedding_dim). If original size is 1-d or 2-d, return it as is.

allennlp.nn.util.viterbi_decode(tag_sequence: torch.Tensor, transition_matrix: torch.Tensor, tag_observations: Union[List[int], NoneType] = None, allowed_start_transitions: torch.Tensor = None, allowed_end_transitions: torch.Tensor = None)[source]

Perform Viterbi decoding in log space over a sequence given a transition matrix specifying pairwise (transition) potentials between tags and a matrix of shape (sequence_length, num_tags) specifying unary potentials for possible tags per timestep.

tag_sequencetorch.Tensor, required.

A tensor of shape (sequence_length, num_tags) representing scores for a set of tags over a given sequence.

transition_matrixtorch.Tensor, required.

A tensor of shape (num_tags, num_tags) representing the binary potentials for transitioning between a given pair of tags.

tag_observationsOptional[List[int]], optional, (default = None)

A list of length sequence_length containing the class ids of observed elements in the sequence, with unobserved elements being set to -1. Note that it is possible to provide evidence which results in degenerate labelings if the sequences of tags you provide as evidence cannot transition between each other, or those transitions are extremely unlikely. In this situation we log a warning, but the responsibility for providing self-consistent evidence ultimately lies with the user.

allowed_start_transitionstorch.Tensor, optional, (default = None)

An optional tensor of shape (num_tags,) describing which tags the START token may transition to. If provided, additional transition constraints will be used for determining the start element of the sequence.

allowed_end_transitionstorch.Tensor, optional, (default = None)

An optional tensor of shape (num_tags,) describing which tags may transition to the end tag. If provided, additional transition constraints will be used for determining the end element of the sequence.


The tag indices of the maximum likelihood tag sequence.


The score of the viterbi path.

allennlp.nn.util.weighted_sum(matrix: torch.Tensor, attention: torch.Tensor) → torch.Tensor[source]

Takes a matrix of vectors and a set of weights over the rows in the matrix (which we call an “attention” vector), and returns a weighted sum of the rows in the matrix. This is the typical computation performed after an attention mechanism.

Note that while we call this a “matrix” of vectors and an attention “vector”, we also handle higher-order tensors. We always sum over the second-to-last dimension of the “matrix”, and we assume that all dimensions in the “matrix” prior to the last dimension are matched in the “vector”. Non-matched dimensions in the “vector” must be directly after the batch dimension.

For example, say I have a “matrix” with dimensions (batch_size, num_queries, num_words, embedding_dim). The attention “vector” then must have at least those dimensions, and could have more. Both:

  • (batch_size, num_queries, num_words) (distribution over words for each query)

  • (batch_size, num_documents, num_queries, num_words) (distribution over words in a query for each document)

are valid input “vectors”, producing tensors of shape: (batch_size, num_queries, embedding_dim) and (batch_size, num_documents, num_queries, embedding_dim) respectively.