Losses

sentence_transformers.losses defines different loss functions that can be used to fine-tune embedding models on training data. The choice of loss function plays a critical role when fine-tuning the model. It determines how well our embedding model will work for the specific downstream task.

Sadly, there is no “one size fits all” loss function. Which loss function is suitable depends on the available training data and on the target task. Consider checking out the Loss Overview to help narrow down your choice of loss function(s).

BatchAllTripletLoss

class sentence_transformers.losses.BatchAllTripletLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, distance_metric=<function BatchHardTripletLossDistanceFunction.eucledian_distance>, margin: float = 5)

BatchAllTripletLoss takes a batch with (sentence, label) pairs and computes the loss for all possible, valid triplets, i.e., anchor and positive must have the same label, anchor and negative a different label. The labels must be integers, with same label indicating sentences from the same class. Your train dataset must contain at least 2 examples per label class.

Parameters
  • model – SentenceTransformer model

  • distance_metric – Function that returns a distance between two embeddings. The class SiameseDistanceMetric contains pre-defined metrics that can be used.

  • margin – Negative samples should be at least margin further apart from the anchor than the positive.

References:
Requirements:
  1. Each sentence must be labeled with a class.

  2. Your dataset must contain at least 2 examples per labels class.

Relations:
  • BatchHardTripletLoss uses only the hardest positive and negative samples, rather than all possible, valid triplets.

  • BatchHardSoftMarginTripletLoss uses only the hardest positive and negative samples, rather than all possible, valid triplets. Also, it does not require setting a margin.

  • BatchSemiHardTripletLoss uses only semi-hard triplets, valid triplets, rather than all possible, valid triplets.

Inputs:

Texts

Labels

single sentences

class

Example:
from sentence_transformers import SentenceTransformer, losses
from sentence_transformers.readers import InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('distilbert-base-nli-mean-tokens')
train_examples = [
    InputExample(texts=['Sentence from class 0'], label=0),
    InputExample(texts=['Another sentence from class 0'], label=0),
    InputExample(texts=['Sentence from class 1'], label=1),
    InputExample(texts=['Sentence from class 2'], label=2),
]
train_batch_size = 2
train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=train_batch_size)
train_loss = losses.BatchAllTripletLoss(model=model)
model.fit(
    train_objectives=[(train_dataloader, train_loss)],
    epochs=10,
)

BatchHardSoftMarginTripletLoss

class sentence_transformers.losses.BatchHardSoftMarginTripletLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, distance_metric=<function BatchHardTripletLossDistanceFunction.eucledian_distance>)

BatchHardSoftMarginTripletLoss takes a batch with (sentence, label) pairs and computes the loss for all possible, valid triplets, i.e., anchor and positive must have the same label, anchor and negative a different label. The labels must be integers, with same label indicating sentences from the same class. Your train dataset must contain at least 2 examples per label class. This soft-margin variant does not require setting a margin.

Parameters
  • model – SentenceTransformer model

  • distance_metric – Function that returns a distance between two embeddings. The class SiameseDistanceMetric contains pre-defined metrics that can be used.

Definitions:
Easy triplets

Triplets which have a loss of 0 because distance(anchor, positive) + margin < distance(anchor, negative).

Hard triplets

Triplets where the negative is closer to the anchor than the positive, i.e., distance(anchor, negative) < distance(anchor, positive).

Semi-hard triplets

Triplets where the negative is not closer to the anchor than the positive, but which still have a positive loss, i.e., distance(anchor, positive) < distance(anchor, negative) + margin.

References:
Requirements:
  1. Each sentence must be labeled with a class.

  2. Your dataset must contain at least 2 examples per labels class.

  3. Your dataset should contain hard positives and negatives.

Relations:
  • BatchHardTripletLoss uses a user-specified margin, while this loss does not require setting a margin.

Inputs:

Texts

Labels

single sentences

class

Example:
from sentence_transformers import SentenceTransformer, losses
from sentence_transformers.readers import InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('distilbert-base-nli-mean-tokens')
train_examples = [
    InputExample(texts=['Sentence from class 0'], label=0),
    InputExample(texts=['Another sentence from class 0'], label=0),
    InputExample(texts=['Sentence from class 1'], label=1),
    InputExample(texts=['Sentence from class 2'], label=2)
]
train_batch_size = 2
train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=train_batch_size)
train_loss = losses.BatchHardSoftMarginTripletLoss(model=model)
model.fit(
    train_objectives=[(train_dataloader, train_loss)],
    epochs=10,
)

BatchHardTripletLoss

class sentence_transformers.losses.BatchHardTripletLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, distance_metric=<function BatchHardTripletLossDistanceFunction.eucledian_distance>, margin: float = 5)

BatchHardTripletLoss takes a batch with (sentence, label) pairs and computes the loss for all possible, valid triplets, i.e., anchor and positive must have the same label, anchor and negative a different label. It then looks for the hardest positive and the hardest negatives. The labels must be integers, with same label indicating sentences from the same class. Your train dataset must contain at least 2 examples per label class.

Parameters
  • model – SentenceTransformer model

  • distance_metric – Function that returns a distance between two embeddings. The class SiameseDistanceMetric contains pre-defined metrics that can be used

  • margin – Negative samples should be at least margin further apart from the anchor than the positive.

Definitions:
Easy triplets

Triplets which have a loss of 0 because distance(anchor, positive) + margin < distance(anchor, negative).

Hard triplets

Triplets where the negative is closer to the anchor than the positive, i.e., distance(anchor, negative) < distance(anchor, positive).

Semi-hard triplets

Triplets where the negative is not closer to the anchor than the positive, but which still have a positive loss, i.e., distance(anchor, positive) < distance(anchor, negative) + margin.

References:
Requirements:
  1. Each sentence must be labeled with a class.

  2. Your dataset must contain at least 2 examples per labels class.

  3. Your dataset should contain hard positives and negatives.

Inputs:

Texts

Labels

single sentences

class

Relations:
Example:
from sentence_transformers import SentenceTransformer, losses
from sentence_transformers.readers import InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('distilbert-base-nli-mean-tokens')
train_examples = [
    InputExample(texts=['Sentence from class 0'], label=0),
    InputExample(texts=['Another sentence from class 0'], label=0),
    InputExample(texts=['Sentence from class 1'], label=1),
    InputExample(texts=['Sentence from class 2'], label=2)
]
train_batch_size = 2
train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=train_batch_size)
train_loss = losses.BatchHardTripletLoss(model=model)
model.fit(
    train_objectives=[(train_dataloader, train_loss)],
    epochs=10,
)

BatchSemiHardTripletLoss

class sentence_transformers.losses.BatchSemiHardTripletLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, distance_metric=<function BatchHardTripletLossDistanceFunction.eucledian_distance>, margin: float = 5)

BatchSemiHardTripletLoss takes a batch with (label, sentence) pairs and computes the loss for all possible, valid triplets, i.e., anchor and positive must have the same label, anchor and negative a different label. It then looks for the semi hard positives and negatives. The labels must be integers, with same label indicating sentences from the same class. Your train dataset must contain at least 2 examples per label class.

Parameters
  • model – SentenceTransformer model

  • distance_metric – Function that returns a distance between two embeddings. The class SiameseDistanceMetric contains pre-defined metrics that can be used

  • margin – Negative samples should be at least margin further apart from the anchor than the positive.

Definitions:
Easy triplets

Triplets which have a loss of 0 because distance(anchor, positive) + margin < distance(anchor, negative).

Hard triplets

Triplets where the negative is closer to the anchor than the positive, i.e., distance(anchor, negative) < distance(anchor, positive).

Semi-hard triplets

Triplets where the negative is not closer to the anchor than the positive, but which still have a positive loss, i.e., distance(anchor, positive) < distance(anchor, negative) + margin.

References:
Requirements:
  1. Each sentence must be labeled with a class.

  2. Your dataset must contain at least 2 examples per labels class.

  3. Your dataset should contain semi hard positives and negatives.

Relations:
  • BatchHardTripletLoss uses only the hardest positive and negative samples, rather than only semi hard positive and negatives.

  • BatchAllTripletLoss uses all possible, valid triplets, rather than only semi hard positive and negatives.

  • BatchHardSoftMarginTripletLoss uses only the hardest positive and negative samples, rather than only semi hard positive and negatives.

Also, it does not require setting a margin.

Inputs:

Texts

Labels

single sentences

class

Example:
from sentence_transformers import SentenceTransformer, losses
from sentence_transformers.readers import InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('distilbert-base-nli-mean-tokens')
train_examples = [
    InputExample(texts=['Sentence from class 0'], label=0),
    InputExample(texts=['Another sentence from class 0'], label=0),
    InputExample(texts=['Sentence from class 1'], label=1),
    InputExample(texts=['Sentence from class 2'], label=2)
]
train_batch_size = 2
train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=train_batch_size)
train_loss = losses.BatchSemiHardTripletLoss(model=model)
model.fit(
    train_objectives=[(train_dataloader, train_loss)],
    epochs=10,
)

ContrastiveLoss

class sentence_transformers.losses.ContrastiveLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, distance_metric=<function SiameseDistanceMetric.<lambda>>, margin: float = 0.5, size_average: bool = True)

Contrastive loss. Expects as input two texts and a label of either 0 or 1. If the label == 1, then the distance between the two embeddings is reduced. If the label == 0, then the distance between the embeddings is increased.

Parameters
  • model – SentenceTransformer model

  • distance_metric – Function that returns a distance between two embeddings. The class SiameseDistanceMetric contains pre-defined metrices that can be used

  • margin – Negative samples (label == 0) should have a distance of at least the margin value.

  • size_average – Average by the size of the mini-batch.

References:
Requirements:
  1. (anchor, positive/negative) pairs

Relations:

It often yields better results.

Inputs:

Texts

Labels

(anchor, positive/negative) pairs

1 if positive, 0 if negative

Example:
from sentence_transformers import SentenceTransformer, losses
from sentence_transformers.readers import InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('all-MiniLM-L6-v2')
train_examples = [
    InputExample(texts=['This is a positive pair', 'Where the distance will be minimized'], label=1),
    InputExample(texts=['This is a negative pair', 'Their distance will be increased'], label=0),
]

train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=2)
train_loss = losses.ContrastiveLoss(model=model)

model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

OnlineContrastiveLoss

class sentence_transformers.losses.OnlineContrastiveLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, distance_metric=<function SiameseDistanceMetric.<lambda>>, margin: float = 0.5)

This Online Contrastive loss is similar to ConstrativeLoss, but it selects hard positive (positives that are far apart) and hard negative pairs (negatives that are close) and computes the loss only for these pairs. This loss often yields better performances than ContrastiveLoss.

Parameters
  • model – SentenceTransformer model

  • distance_metric – Function that returns a distance between two embeddings. The class SiameseDistanceMetric contains pre-defined metrics that can be used

  • margin – Negative samples (label == 0) should have a distance of at least the margin value.

References:
Requirements:
  1. (anchor, positive/negative) pairs

  2. Data should include hard positives and hard negatives

Relations:
  • ContrastiveLoss is similar, but does not use hard positive and hard negative pairs.

OnlineContrastiveLoss often yields better results.

Inputs:

Texts

Labels

(anchor, positive/negative) pairs

1 if positive, 0 if negative

Example:
from sentence_transformers import SentenceTransformer, losses, InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('all-MiniLM-L6-v2')
train_examples = [
    InputExample(texts=['This is a positive pair', 'Where the distance will be minimized'], label=1),
    InputExample(texts=['This is a negative pair', 'Their distance will be increased'], label=0),
]

train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=2)
train_loss = losses.OnlineContrastiveLoss(model=model)
model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

ContrastiveTensionLoss

class sentence_transformers.losses.ContrastiveTensionLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer)

This loss expects only single sentences, without any labels. Positive and negative pairs are automatically created via random sampling, such that a positive pair consists of two identical sentences and a negative pair consists of two different sentences. An independent copy of the encoder model is created, which is used for encoding the first sentence of each pair. The original encoder model encodes the second sentence. The embeddings are compared and scored using the generated labels (1 if positive, 0 if negative) using the binary cross entropy objective.

Note that you must use the ContrastiveTensionDataLoader for this loss. The pos_neg_ratio of the ContrastiveTensionDataLoader can be used to determine the number of negative pairs per positive pair.

Generally, ContrastiveTensionLossInBatchNegatives is recommended over this loss, as it gives a stronger training signal.

Parameters

model – SentenceTransformer model

References:
Relations:
Inputs:

Texts

Labels

single sentences

none

Example:
from sentence_transformers import SentenceTransformer, losses
from sentence_transformers.losses import ContrastiveTensionDataLoader

model = SentenceTransformer('all-MiniLM-L6-v2')
train_examples = [
    'This is the 1st sentence',
    'This is the 2nd sentence',
    'This is the 3rd sentence',
    'This is the 4th sentence',
    'This is the 5th sentence',
    'This is the 6th sentence',
    'This is the 7th sentence',
    'This is the 8th sentence',
    'This is the 9th sentence',
    'This is the final sentence',
]

train_dataloader = ContrastiveTensionDataLoader(train_examples, batch_size=3, pos_neg_ratio=3)
train_loss = losses.ContrastiveTensionLoss(model=model)

model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

Initializes internal Module state, shared by both nn.Module and ScriptModule.

ContrastiveTensionLossInBatchNegatives

class sentence_transformers.losses.ContrastiveTensionLossInBatchNegatives(model: sentence_transformers.SentenceTransformer.SentenceTransformer, scale: float = 20.0, similarity_fct=<function cos_sim>)

This loss expects only single sentences, without any labels. Positive and negative pairs are automatically created via random sampling, such that a positive pair consists of two identical sentences and a negative pair consists of two different sentences. An independent copy of the encoder model is created, which is used for encoding the first sentence of each pair. The original encoder model encodes the second sentence. Unlike ContrastiveTensionLoss, this loss uses the batch negative sampling strategy, i.e. the negative pairs are sampled from the batch. Using in-batch negative sampling gives a stronger training signal than the original ContrastiveTensionLoss. The performance usually increases with increasing batch sizes.

Note that you should not use the ContrastiveTensionDataLoader for this loss, but just a normal DataLoader with InputExample instances. The two texts of each InputExample instance should be identical.

Parameters
  • model – SentenceTransformer model

  • scale – Output of similarity function is multiplied by scale value

  • similarity_fct – similarity function between sentence embeddings. By default, cos_sim. Can also be set to dot product (and then set scale to 1)

References:
Relations:
  • ContrastiveTensionLoss does not select negative pairs in-batch, resulting in a weaker training signal than this loss.

Inputs:

Texts

Labels

(anchor, anchor) pairs

none

Example:
from sentence_transformers import SentenceTransformer, losses
from torch.utils.data import DataLoader

model = SentenceTransformer('all-MiniLM-L6-v2')
train_examples = [
    InputExample(texts=['This is a positive pair', 'Where the distance will be minimized'], label=1),
    InputExample(texts=['This is a negative pair', 'Their distance will be increased'], label=0),
]
train_examples = [
    InputExample(texts=['This is the 1st sentence', 'This is the 1st sentence']),
    InputExample(texts=['This is the 2nd sentence', 'This is the 2nd sentence']),
    InputExample(texts=['This is the 3rd sentence', 'This is the 3rd sentence']),
    InputExample(texts=['This is the 4th sentence', 'This is the 4th sentence']),
    InputExample(texts=['This is the 5th sentence', 'This is the 5th sentence']),
]

train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=32)
train_loss = losses.ContrastiveTensionLossInBatchNegatives(model=model)

model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

CoSENTLoss

class sentence_transformers.losses.CoSENTLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, scale: float = 20.0, similarity_fct=<function pairwise_cos_sim>)

This class implements CoSENT (Cosine Sentence) loss. It expects that each of the InputExamples consists of a pair of texts and a float valued label, representing the expected similarity score between the pair.

It computes the following loss function:

loss = logsum(1+exp(s(k,l)-s(i,j))+exp...), where (i,j) and (k,l) are any of the input pairs in the batch such that the expected similarity of (i,j) is greater than (k,l). The summation is over all possible pairs of input pairs in the batch that match this condition.

Anecdotal experiments show that this loss function produces a more powerful training signal than CosineSimilarityLoss, resulting in faster convergence and a final model with superior performance. Consequently, CoSENTLoss may be used as a drop-in replacement for CosineSimilarityLoss in any training script.

Parameters
  • model – SentenceTransformerModel

  • similarity_fct – Function to compute the PAIRWISE similarity between embeddings. Default is util.pairwise_cos_sim.

  • scale – Output of similarity function is multiplied by scale value. Represents the inverse temperature.

References:
Requirements:
  • Sentence pairs with corresponding similarity scores in range of the similarity function. Default is [-1,1].

Relations:
  • AnglELoss is CoSENTLoss with pairwise_angle_sim as the metric, rather than pairwise_cos_sim.

  • CosineSimilarityLoss seems to produce a weaker training signal than CoSENTLoss. In our experiments, CoSENTLoss is recommended.

Inputs:

Texts

Labels

(sentence_A, sentence_B) pairs

float similarity score

Example:
from sentence_transformers import SentenceTransformer, losses
from sentence_transformers.readers import InputExample

model = SentenceTransformer('bert-base-uncased')
train_examples = [InputExample(texts=['My first sentence', 'My second sentence'], label=1.0),
        InputExample(texts=['My third sentence', 'Unrelated sentence'], label=0.3)]

train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=train_batch_size)
train_loss = losses.CoSENTLoss(model=model)

AnglELoss

class sentence_transformers.losses.AnglELoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, scale: float = 20.0)

This class implements AnglE (Angle Optimized) loss. This is a modification of CoSENTLoss, designed to address the following issue: The cosine function’s gradient approaches 0 as the wave approaches the top or bottom of its form. This can hinder the optimization process, so AnglE proposes to instead optimize the angle difference in complex space in order to mitigate this effect.

It expects that each of the InputExamples consists of a pair of texts and a float valued label, representing the expected similarity score between the pair.

It computes the following loss function:

loss = logsum(1+exp(s(k,l)-s(i,j))+exp...), where (i,j) and (k,l) are any of the input pairs in the batch such that the expected similarity of (i,j) is greater than (k,l). The summation is over all possible pairs of input pairs in the batch that match this condition. This is the same as CoSENTLoss, with a different similarity function.

Parameters
  • model – SentenceTransformerModel

  • scale – Output of similarity function is multiplied by scale value. Represents the inverse temperature.

References:
Requirements:
  • Sentence pairs with corresponding similarity scores in range of the similarity function. Default is [-1,1].

Relations:
  • CoSENTLoss is AnglELoss with pairwise_cos_sim as the metric, rather than pairwise_angle_sim.

  • CosineSimilarityLoss seems to produce a weaker training signal than CoSENTLoss or AnglELoss.

Inputs:

Texts

Labels

(sentence_A, sentence_B) pairs

float similarity score

Example:
from sentence_transformers import SentenceTransformer, losses
from sentence_transformers.readers import InputExample

model = SentenceTransformer('bert-base-uncased')
train_examples = [InputExample(texts=['My first sentence', 'My second sentence'], label=1.0),
        InputExample(texts=['My third sentence', 'Unrelated sentence'], label=0.3)]

train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=train_batch_size)
train_loss = losses.AnglELoss(model=model)

CosineSimilarityLoss

SBERT Siamese Network Architecture

For each sentence pair, we pass sentence A and sentence B through our network which yields the embeddings u und v. The similarity of these embeddings is computed using cosine similarity and the result is compared to the gold similarity score.

This allows our network to be fine-tuned to recognize the similarity of sentences.

class sentence_transformers.losses.CosineSimilarityLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, loss_fct=MSELoss(), cos_score_transformation=Identity())

CosineSimilarityLoss expects that the InputExamples consists of two texts and a float label. It computes the vectors u = model(sentence_A) and v = model(sentence_B) and measures the cosine-similarity between the two. By default, it minimizes the following loss: ||input_label - cos_score_transformation(cosine_sim(u,v))||_2.

Parameters
  • model – SentenceTransformer model

  • loss_fct – Which pytorch loss function should be used to compare the cosine_similarity(u, v) with the input_label? By default, MSE is used: ||input_label - cosine_sim(u, v)||_2

  • cos_score_transformation – The cos_score_transformation function is applied on top of cosine_similarity. By default, the identify function is used (i.e. no change).

References:
Requirements:
  1. Sentence pairs with corresponding similarity scores in range [0, 1]

Relations:
  • CoSENTLoss seems to produce a stronger training signal than CosineSimilarityLoss. In our experiments, CoSENTLoss is recommended.

  • AnglELoss is CoSENTLoss with pairwise_angle_sim as the metric, rather than pairwise_cos_sim. It also produces a stronger training signal than CosineSimilarityLoss.

Inputs:

Texts

Labels

(sentence_A, sentence_B) pairs

float similarity score

Example:
from sentence_transformers import SentenceTransformer, InputExample, losses
from torch.utils.data import DataLoader

model = SentenceTransformer('distilbert-base-nli-mean-tokens')
train_examples = [
    InputExample(texts=['My first sentence', 'My second sentence'], label=0.8),
    InputExample(texts=['Another pair', 'Unrelated sentence'], label=0.3)
]
train_batch_size = 1
train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=train_batch_size)
train_loss = losses.CosineSimilarityLoss(model=model)

model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

DenoisingAutoEncoderLoss

class sentence_transformers.losses.DenoisingAutoEncoderLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, decoder_name_or_path: Optional[str] = None, tie_encoder_decoder: bool = True)

This loss expects as input a pairs of damaged sentences and the corresponding original ones. During training, the decoder reconstructs the original sentences from the encoded sentence embeddings. Here the argument ‘decoder_name_or_path’ indicates the pretrained model (supported by Hugging Face) to be used as the decoder. Since decoding process is included, here the decoder should have a class called XXXLMHead (in the context of Hugging Face’s Transformers). The ‘tie_encoder_decoder’ flag indicates whether to tie the trainable parameters of encoder and decoder, which is shown beneficial to model performance while limiting the amount of required memory. Only when the encoder and decoder are from the same architecture, can the flag ‘tie_encoder_decoder’ work.

The data generation process (i.e. the ‘damaging’ process) has already been implemented in DenoisingAutoEncoderDataset, allowing you to only provide regular sentences.

Parameters
  • model – SentenceTransformer model

  • decoder_name_or_path – Model name or path for initializing a decoder (compatible with Huggingface’s Transformers)

  • tie_encoder_decoder – whether to tie the trainable parameters of encoder and decoder

References:
Requirements:
  1. The decoder should have a class called XXXLMHead (in the context of Hugging Face’s Transformers)

  2. Should use a large corpus

Inputs:

Texts

Labels

(damaged_sentence, original_sentence) pairs

none

sentence fed through DenoisingAutoEncoderDataset

none

Example:
from sentence_transformers import SentenceTransformer, losses
from sentence_transformers.datasets import DenoisingAutoEncoderDataset
from torch.utils.data import DataLoader

model_name = "bert-base-cased"
model = SentenceTransformer(model_name)
train_sentences = [
    "First training sentence", "Second training sentence", "Third training sentence", "Fourth training sentence",
]
batch_size = 2
train_dataset = DenoisingAutoEncoderDataset(train_sentences)
train_dataloader = DataLoader(train_dataset, batch_size=batch_size, shuffle=True, drop_last=True)
train_loss = losses.DenoisingAutoEncoderLoss(
    model, decoder_name_or_path=model_name, tie_encoder_decoder=True
)
model.fit(
    train_objectives=[(train_dataloader, train_loss)],
    epochs=10,
)

MSELoss

class sentence_transformers.losses.MSELoss(model)

Computes the MSE loss between the computed sentence embedding and a target sentence embedding. This loss is used when extending sentence embeddings to new languages as described in our publication Making Monolingual Sentence Embeddings Multilingual using Knowledge Distillation.

For an example, see the distillation documentation on extending language models to new languages.

Parameters

model – SentenceTransformerModel

References:
Requirements:
  1. Usually uses a finetuned teacher M in a knowledge distillation setup

Relations:
  • MarginMSELoss is equivalent to this loss, but with a margin through a negative pair.

Input:

Texts

Labels

single sentences

model sentence embeddings

Example:

from sentence_transformers import SentenceTransformer, InputExample, losses
from torch.utils.data import DataLoader

model_en = SentenceTransformer('bert-base-cased')
model_fr = SentenceTransformer('flaubert/flaubert_base_cased')

examples_en = ['The first sentence',  'The second sentence', 'The third sentence',  'The fourth sentence']
examples_fr = ['La première phrase',  'La deuxième phrase', 'La troisième phrase',  'La quatrième phrase']
train_batch_size = 2

labels_en_en = model_en.encode(examples_en)
examples_en_fr = [InputExample(texts=[x], label=labels_en_en[i]) for i, x in enumerate(examples_en)]
loader_en_fr = DataLoader(examples_en_fr, batch_size=train_batch_size)

examples_fr_fr = [InputExample(texts=[x], label=labels_en_en[i]) for i, x in enumerate(examples_fr)]
loader_fr_fr = DataLoader(examples_fr_fr, batch_size=train_batch_size)

train_loss = losses.MSELoss(model=model_fr)
model_fr.fit(
    [(loader_en_fr, train_loss), (loader_fr_fr, train_loss)],
    epochs=10,
)

MarginMSELoss

class sentence_transformers.losses.MarginMSELoss(model, similarity_fct=<function pairwise_dot_score>)

Compute the MSE loss between the |sim(Query, Pos) - sim(Query, Neg)| and |gold_sim(Query, Pos) - gold_sim(Query, Neg)|. By default, sim() is the dot-product. The gold_sim is often the similarity score from a teacher model.

In contrast to MultipleNegativesRankingLoss, the two passages do not have to be strictly positive and negative, both can be relevant or not relevant for a given query. This can be an advantage of MarginMSELoss over MultipleNegativesRankingLoss, but note that the MarginMSELoss is much slower to train. With MultipleNegativesRankingLoss, with a batch size of 64, we compare one query against 128 passages. With MarginMSELoss, we compare a query only against two passages.

Parameters
  • model – SentenceTransformerModel

  • similarity_fct – Which similarity function to use.

References:
Requirements:
  1. (query, passage_one, passage_two) triplets

  2. Usually used with a finetuned teacher M in a knowledge distillation setup

Relations:
  • MSELoss is equivalent to this loss, but without a margin through the negative pair.

Inputs:

Texts

Labels

(query, passage_one, passage_two) triplets

M(query, passage_one) - M(query, passage_two)

Example:
from sentence_transformers import SentenceTransformer, InputExample, losses
from sentence_transformers.util import pairwise_dot_score
from torch.utils.data import DataLoader
import torch

student_model = SentenceTransformer('sentence-transformers/distilbert-base-nli-mean-tokens')
teacher_model = SentenceTransformer('sentence-transformers/bert-base-nli-stsb-mean-tokens')

train_examples = [
    ['The first query',  'The first positive passage',  'The first negative passage'],
    ['The second query', 'The second positive passage', 'The second negative passage'],
    ['The third query',  'The third positive passage',  'The third negative passage'],
]
train_batch_size = 1
encoded = torch.tensor([teacher_model.encode(x).tolist() for x in train_examples])
labels = pairwise_dot_score(encoded[:, 0], encoded[:, 1]) - pairwise_dot_score(encoded[:, 0], encoded[:, 2])

train_input_examples = [InputExample(texts=x, label=labels[i]) for i, x in enumerate(train_examples)]
train_dataloader = DataLoader(train_input_examples, shuffle=True, batch_size=train_batch_size)
train_loss = losses.MarginMSELoss(model=student_model)

student_model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

MatryoshkaLoss

class sentence_transformers.losses.MatryoshkaLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, loss: torch.nn.modules.module.Module, matryoshka_dims: List[int], matryoshka_weights: Optional[List[Union[float, int]]] = None)

The MatryoshkaLoss can be seen as a loss modifier that allows you to use other loss functions at various different embedding dimensions. This is useful for when you want to train a model where users have the option to lower the embedding dimension to improve their embedding comparison speed and costs.

Parameters
  • model – SentenceTransformer model

  • loss – The loss function to be used, e.g. MultipleNegativesRankingLoss, CoSENTLoss, etc.

  • matryoshka_dims – A list of embedding dimensions to be used for the loss function, e.g. [768, 512, 256, 128, 64].

  • matryoshka_weights – A list of weights to be used for the loss function, e.g. [1, 1, 1, 1, 1]. If None, then the weights will be set to 1 for all dimensions.

References:
Requirements:
  1. The base loss cannot be CachedMultipleNegativesRankingLoss.

Input:

Texts

Labels

any

any

Example:
from sentence_transformers import SentenceTransformer, losses, InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('microsoft/mpnet-base')
train_examples = [
    InputExample(texts=['Anchor 1', 'Positive 1']),
    InputExample(texts=['Anchor 2', 'Positive 2']),
]
train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=32)
train_loss = losses.MultipleNegativesRankingLoss(model=model)
train_loss = losses.MatryoshkaLoss(model, train_loss, [768, 512, 256, 128, 64])
model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

MegaBatchMarginLoss

class sentence_transformers.losses.MegaBatchMarginLoss(model, positive_margin: float = 0.8, negative_margin: float = 0.3, use_mini_batched_version: bool = True, mini_batch_size: int = 50)

Given a large batch (like 500 or more examples) of (anchor_i, positive_i) pairs, find for each pair in the batch the hardest negative, i.e. find j != i such that cos_sim(anchor_i, positive_j) is maximal. Then create from this a triplet (anchor_i, positive_i, positive_j) where positive_j serves as the negative for this triplet.

Then train as with the triplet loss.

Parameters
  • model – SentenceTransformerModel

  • positive_margin – Positive margin, cos(anchor, positive) should be > positive_margin

  • negative_margin – Negative margin, cos(anchor, negative) should be < negative_margin

  • use_mini_batched_version – As large batch sizes require a lot of memory, we can use a mini-batched version. We break down the large batch into smaller batches with fewer examples.

  • mini_batch_size – Size for the mini-batches. Should be a devisor for the batch size in your data loader.

References:
Requirements:
  1. (anchor, positive) pairs

  2. Large batches (500 or more examples)

Input:

Texts

Labels

(anchor, positive) pairs

none

Example:
from sentence_transformers import SentenceTransformer, InputExample, losses
from torch.utils.data import DataLoader

model = SentenceTransformer('all-MiniLM-L6-v2')

total_examples = 500
train_batch_size = 250
train_mini_batch_size = 32

train_examples = [
    InputExample(texts=[f"This is sentence number {i}", f"This is sentence number {i+1}"]) for i in range(total_examples)
]
train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=train_batch_size)
train_loss = losses.MegaBatchMarginLoss(model=model, mini_batch_size=train_mini_batch_size)

model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

MultipleNegativesRankingLoss

MultipleNegativesRankingLoss is a great loss function if you only have positive pairs, for example, only pairs of similar texts like pairs of paraphrases, pairs of duplicate questions, pairs of (query, response), or pairs of (source_language, target_language).

class sentence_transformers.losses.MultipleNegativesRankingLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, scale: float = 20.0, similarity_fct=<function cos_sim>)

This loss expects as input a batch consisting of sentence pairs (a_1, p_1), (a_2, p_2)..., (a_n, p_n) where we assume that (a_i, p_i) are a positive pair and (a_i, p_j) for i != j a negative pair.

For each a_i, it uses all other p_j as negative samples, i.e., for a_i, we have 1 positive example (p_i) and n-1 negative examples (p_j). It then minimizes the negative log-likehood for softmax normalized scores.

This loss function works great to train embeddings for retrieval setups where you have positive pairs (e.g. (query, relevant_doc)) as it will sample in each batch n-1 negative docs randomly.

The performance usually increases with increasing batch sizes.

You can also provide one or multiple hard negatives per anchor-positive pair by structering the data like this: (a_1, p_1, n_1), (a_2, p_2, n_2). Then, n_1 is a hard negative for (a_1, p_1). The loss will use for the pair (a_i, p_i) all p_j for j != i and all n_j as negatives.

Parameters
  • model – SentenceTransformer model

  • scale – Output of similarity function is multiplied by scale value

  • similarity_fct – similarity function between sentence embeddings. By default, cos_sim. Can also be set to dot product (and then set scale to 1)

References:
Requirements:
  1. (anchor, positive) pairs or (anchor, positive, negative) triplets

Relations:

much higher batch sizes (and thus better performance) without extra memory usage. However, it requires more training time. - MultipleNegativesSymmetricRankingLoss is equivalent to this loss, but with an additional loss term.

Inputs:

Texts

Labels

(anchor, positive) pairs

none

(anchor, positive, negative) triplets

none

Example:
from sentence_transformers import SentenceTransformer, losses, InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('distilbert-base-uncased')
train_examples = [
    InputExample(texts=['Anchor 1', 'Positive 1']),
    InputExample(texts=['Anchor 2', 'Positive 2']),
]
train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=32)
train_loss = losses.MultipleNegativesRankingLoss(model=model)
model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

CachedMultipleNegativesRankingLoss

class sentence_transformers.losses.CachedMultipleNegativesRankingLoss(model: SentenceTransformer, scale: float = 20.0, similarity_fct: callable[[Tensor, Tensor], Tensor] = <function cos_sim>, mini_batch_size: int = 32, show_progress_bar: bool = False)

Boosted version of MultipleNegativesRankingLoss (https://arxiv.org/pdf/1705.00652.pdf) by GradCache (https://arxiv.org/pdf/2101.06983.pdf).

Constrastive learning (here our MNRL loss) with in-batch negatives is usually hard to work with large batch sizes due to (GPU) memory limitation. Even with batch-scaling methods like gradient-scaling, it cannot work either. This is because the in-batch negatives make the data points within the same batch non-independent and thus the batch cannot be broke down into mini-batches. GradCache is a smart way to solve this problem. It achieves the goal by dividing the computation into two stages of embedding and loss calculation, which both can be scaled by mini-batches. As a result, memory of constant size (e.g. that works with batch size = 32) can now process much larger batches (e.g. 65536).

In detail:

  1. It first does a quick embedding step without gradients/computation graphs to get all the embeddings;

  2. Calculate the loss, backward up to the embeddings and cache the gradients wrt. to the embeddings;

  3. A 2nd embedding step with gradients/computation graphs and connect the cached gradients into the backward chain.

Notes: All steps are done with mini-batches. In the original implementation of GradCache, (2) is not done in mini-batches and requires a lot memory when batch size large. One drawback is about the speed. GradCache will sacrifice around 20% computation time according to the paper.

Parameters
  • model – SentenceTransformer model

  • scale – Output of similarity function is multiplied by scale value

  • similarity_fct – similarity function between sentence embeddings. By default, cos_sim. Can also be set to dot product (and then set scale to 1)

References:
Requirements:
  1. (anchor, positive) pairs or (anchor, positive, negative pairs)

  2. Should be used with large batch sizes for superior performance, but has slower training time than MultipleNegativesRankingLoss

Relations:

(and thus better performance) without extra memory usage. This loss also trains roughly 2x to 2.4x slower than MultipleNegativesRankingLoss.

Inputs:

Texts

Labels

(anchor, positive) pairs

none

(anchor, positive, negative) triplets

none

Example:
from sentence_transformers import SentenceTransformer, losses, InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('distilbert-base-uncased')
train_examples = [
    InputExample(texts=['Anchor 1', 'Positive 1']),
    InputExample(texts=['Anchor 2', 'Positive 2']),
]
train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=1024)  # Here we can try much larger batch sizes!
train_loss = losses.CachedMultipleNegativesRankingLoss(model=model, mini_batch_size = 32)
model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

MultipleNegativesSymmetricRankingLoss

class sentence_transformers.losses.MultipleNegativesSymmetricRankingLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, scale: float = 20.0, similarity_fct=<function cos_sim>)

This loss is an adaptation of MultipleNegativesRankingLoss. MultipleNegativesRankingLoss computes the following loss: For a given anchor and a list of candidates, find the positive candidate.

In MultipleNegativesSymmetricRankingLoss, we add another loss term: Given the positive and a list of all anchors, find the correct (matching) anchor.

For the example of question-answering: You have (question, answer)-pairs. MultipleNegativesRankingLoss just computes the loss to find the answer for a given question. MultipleNegativesSymmetricRankingLoss additionally computes the loss to find the question for a given answer.

Note: If you pass triplets, the negative entry will be ignored. A anchor is just searched for the positive.

Parameters
  • model – SentenceTransformer model

  • scale – Output of similarity function is multiplied by scale value

  • similarity_fct – similarity function between sentence embeddings. By default, cos_sim. Can also be set to dot product (and then set scale to 1)

Requirements:
  1. (anchor, positive) pairs

Relations:
Inputs:

Texts

Labels

(anchor, positive) pairs

none

Example:
from sentence_transformers import SentenceTransformer, losses, InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('distilbert-base-uncased')
train_examples = [
    InputExample(texts=['Anchor 1', 'Positive 1']),
    InputExample(texts=['Anchor 2', 'Positive 2']),
]
train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=32)
train_loss = losses.MultipleNegativesSymmetricRankingLoss(model=model)
model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

SoftmaxLoss

class sentence_transformers.losses.SoftmaxLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, sentence_embedding_dimension: int, num_labels: int, concatenation_sent_rep: bool = True, concatenation_sent_difference: bool = True, concatenation_sent_multiplication: bool = False, loss_fct: Callable = CrossEntropyLoss())

This loss was used in our SBERT publication (https://arxiv.org/abs/1908.10084) to train the SentenceTransformer model on NLI data. It adds a softmax classifier on top of the output of two transformer networks.

MultipleNegativesRankingLoss is an alternative loss function that often yields better results, as per https://arxiv.org/abs/2004.09813.

Parameters
  • model – SentenceTransformer model

  • sentence_embedding_dimension – Dimension of your sentence embeddings

  • num_labels – Number of different labels

  • concatenation_sent_rep – Concatenate vectors u,v for the softmax classifier?

  • concatenation_sent_difference – Add abs(u-v) for the softmax classifier?

  • concatenation_sent_multiplication – Add u*v for the softmax classifier?

  • loss_fct – Optional: Custom pytorch loss function. If not set, uses nn.CrossEntropyLoss()

References:
Requirements:
  1. sentence pairs with a class label

Inputs:

Texts

Labels

(sentence_A, sentence_B) pairs

class

Example:
from sentence_transformers import SentenceTransformer, SentencesDataset, losses
from sentence_transformers.readers import InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('distilbert-base-nli-mean-tokens')
train_examples = [
    InputExample(texts=['First pair, sent A',  'First pair, sent B'], label=0),
    InputExample(texts=['Second pair, sent A', 'Second pair, sent B'], label=1),
    InputExample(texts=['Third pair, sent A',  'Third pair, sent B'], label=0),
    InputExample(texts=['Fourth pair, sent A', 'Fourth pair, sent B'], label=2),
]
train_batch_size = 2
train_dataset = SentencesDataset(train_examples, model)
train_dataloader = DataLoader(train_dataset, shuffle=True, batch_size=train_batch_size)
train_loss = losses.SoftmaxLoss(
    model=model,
    sentence_embedding_dimension=model.get_sentence_embedding_dimension(),
    num_labels=len(set(x.label for x in train_examples))
)
model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)

TripletLoss

class sentence_transformers.losses.TripletLoss(model: sentence_transformers.SentenceTransformer.SentenceTransformer, distance_metric=<function TripletDistanceMetric.<lambda>>, triplet_margin: float = 5)

This class implements triplet loss. Given a triplet of (anchor, positive, negative), the loss minimizes the distance between anchor and positive while it maximizes the distance between anchor and negative. It compute the following loss function:

loss = max(||anchor - positive|| - ||anchor - negative|| + margin, 0).

Margin is an important hyperparameter and needs to be tuned respectively.

Parameters
  • model – SentenceTransformerModel

  • distance_metric – Function to compute distance between two embeddings. The class TripletDistanceMetric contains common distance metrices that can be used.

  • triplet_margin – The negative should be at least this much further away from the anchor than the positive.

References:
Requirements:
  1. (anchor, positive, negative) triplets

Inputs:

Texts

Labels

(anchor, positive, negative) triplets

none

Example:
from sentence_transformers import SentenceTransformer,  SentencesDataset, losses
from sentence_transformers.readers import InputExample
from torch.utils.data import DataLoader

model = SentenceTransformer('distilbert-base-nli-mean-tokens')
train_examples = [
    InputExample(texts=['Anchor 1', 'Positive 1', 'Negative 1']),
    InputExample(texts=['Anchor 2', 'Positive 2', 'Negative 2']),
]
train_batch_size = 1
train_dataset = SentencesDataset(train_examples, model)
train_dataloader = DataLoader(train_dataset, shuffle=True, batch_size=train_batch_size)
train_loss = losses.TripletLoss(model=model)
model.fit(
    [(train_dataloader, train_loss)],
    epochs=10,
)