Zero-shot Classification [Top 6 Models, How To Tutorial In Python & Alternatives]

by | Aug 1, 2023 | Data Science, Machine Learning, Natural Language Processing

What is zero-shot classification?

Zero-shot classification is a machine learning approach in which a model can classify data into multiple classes without any specific training examples for those classes. In traditional supervised learning, a model is trained on a labelled dataset with examples for each category it needs to classify. However, the model can generalize to new classes it has never seen during training in zero-shot classification.

The key idea behind zero-shot classification is using semantic embeddings or representation learning. Models are trained to understand the relationships between different classes and can transfer this understanding to classify unseen classes based on their semantic similarities.

Here’s a general outline of how zero-shot classification works:

  1. Semantic Embeddings: The model is pre-trained on a large dataset, typically using language modelling or image recognition techniques. This pre-training helps the model learn meaningful and continuous representations of words, sentences, or images.
  2. Class Descriptions: A textual description or semantic attribute is provided for each class. These descriptions are often supplied as natural language sentences but can also be embeddings or other structured representations.
  3. Semantic Alignment: The model learns to associate the provided class descriptions with the known semantic representations during training. This alignment helps the model understand the relationship between the text embeddings and the class labels.
  4. Inference: When presented with a new data instance (e.g., an image or a text), the model can perform classification based on the semantic relationships it has learned. It uses the class descriptions and the input data’s semantic embeddings to predict the classes that best match the input.

Zero-shot classification has applications in various domains, including natural language processing (NLP) and computer vision. For instance, a model can be trained in NLP to understand the semantic relationships between words. Then it can perform zero-shot sentiment analysis or topic classification without specific training examples for those sentiments or topics.

Zero-shot classification has applications in various domains, including natural language processing (NLP) and computer vision.

Zero-shot classification has applications in various domains, including natural language processing (NLP) and computer vision.

It is essential to note that the performance of zero-shot classification may not be as accurate as traditional supervised learning, especially for classes dissimilar to the seen classes during pre-training. To improve performance, researchers often use few-shot learning, where a few examples of the new classes are provided, or fine-tuning, where the model is fine-tuned on a limited set of examples from the new classes.

What is a zero-shot classification used for?

Zero-shot classification has various use cases in different domains due to its flexibility and adaptability to handle unseen classes. Some of the everyday use cases of zero-shot classification include:

  1. Topic Classification: Zero-shot classification can categorize text documents into different topics or themes, even if the model has never seen specific examples during training.
  2. Sentiment Analysis: By providing class descriptions like “positive” and “negative,” zero-shot classification can be used to determine the sentiment of a given text without any explicit training examples for specific sentiments.
  3. Product Categorization: E-commerce platforms can use zero-shot classification to automatically classify products into relevant categories, enabling better organization and search functionalities.
  4. Language Identification: Zero-shot classification can be employed to identify the language of a given text, allowing multilingual applications to adapt to different languages dynamically.
  5. Intent Detection in Chatbots: For chatbots and virtual assistants, zero-shot classification can be used to understand user intents without collecting extensive examples for all possible user queries.
  6. Document Type Classification: Zero-shot classification can help automatically categorize different types of documents, such as invoices, contracts, and reports, without the need for specific training data for each document type.
  7. News Article Categorization: Online news portals can utilize zero-shot classification to automatically assign relevant categories to news articles, helping readers find articles of interest more efficiently.
  8. Content Moderation: In content moderation systems, zero-shot classification can help identify and flag inappropriate or harmful content without needing specific training examples for every type of inappropriate content.
  9. Image Classification: Zero-shot classification is not limited to text data; it can also be applied to image classification tasks to categorize images into different classes without explicit training examples for those classes.
  10. Anomaly Detection: Zero-shot classification can detect anomalies in data by assigning a class label of “normal” or “anomaly” without the need for labelled examples of anomalies.

These are just a few examples, and the applications of zero-shot classification can extend to many other domains and tasks. The flexibility of zero-shot learning makes it a valuable tool for scenarios where data constantly evolve, or new classes must be introduced without retraining the model from scratch. However, it’s essential to carefully design the class descriptions and consider the performance trade-offs in real-world applications.

What models can you use for zero-shot text classification?

You can use several pre-trained language models available in the transformers library from Hugging Face for zero-shot text classification. Different models have different architectures and pre-training objectives, which can affect their performance in zero-shot tasks. Here are some popular models that are commonly used for zero-shot classification:

  1. BART: BART (Bidirectional and Auto-Regressive Transformers) is a denoising autoencoder pre-trained on a large corpus. It is beneficial for generating textual data and has shown promising results in zero-shot classification tasks.
  2. T5: T5 (Text-to-Text Transfer Transformer) is a transformer model that frames almost all NLP tasks as text-to-text problems. It can be adapted for zero-shot learning by providing the task description as input alongside the text to classify.
  3. GPT-3: GPT-3 (Generative Pre-trained Transformer 3) is one of the most significant language models available and has impressive zero-shot capabilities. Although GPT-3 might not be directly accessible due to its size, smaller versions and similar models are available.
  4. RoBERTa: RoBERTa (A Robustly Optimized BERT Pre-training Approach) is a variant of BERT that modifies the training process to improve its performance. It is widely used for various NLP tasks, including zero-shot classification.
  5. BERT: BERT (Bidirectional Encoder Representations from Transformers) is one of the pioneering language models for NLP. While not explicitly designed for zero-shot learning, it can still perform reasonably well in zero-shot classification tasks.
  6. ALBERT: ALBERT (A Lite BERT) is a lightweight version of BERT that reduces the model’s size and training time while maintaining performance. It can be a good choice for zero-shot classification in resource-constrained environments.

When using these models for zero-shot classification, you can choose the appropriate one based on factors like model size, performance, and available computational resources. Remember that larger models generally perform better but require more memory and computational power.

Python example of zero-shot text classification

To perform zero-shot text classification in Python, we can use the “zero-shot” functionality provided by the transformers library from Hugging Face. This library includes pre-trained language models like GPT-3 and BERT, described above, which can be used for zero-shot text classification. First, you need to install the transformers library if you haven’t already:

pip install transformers 

Now, let’s go through a Python example of zero-shot text classification using the transformers library:

from transformers import pipeline

# Load the zero-shot classification pipeline with the desired model
# You can choose different models, like 'facebook/bart-large-mnli' or 'roberta-large-mnli'
classifier = pipeline("zero-shot-classification", model="facebook/bart-large-mnli")

# Example class descriptions and input text
class_descriptions = ["politics", "sports", "technology"]
input_text = "The new software update introduces exciting features and improvements."

# Perform zero-shot classification
result = classifier(input_text, class_descriptions)

# Print the results
print("Input Text:", input_text)
for label, score in zip(result["labels"], result["scores"]):
    print(f"{label}: {score:.3f}")

In this example, we use the BART model fine-tuned on the MultiNLI dataset for zero-shot classification. You can experiment with different models the Transformers library provides based on your requirements.

Make sure to use a pre-trained model suitable for zero-shot text classification. Different models may provide different results depending on their architecture and pre-training objectives.

Please note that the performance of zero-shot text classification heavily depends on the quality and relevance of the provided class descriptions. The more informative and accurate the class descriptions, the better the classification results will likely be. You might need to experiment with different class descriptions and models to achieve the best results for your specific use case.

Advantages and disadvantages of zero-shot learning

Zero-shot learning for classification offers several advantages and disadvantages, which are essential when deciding whether to use this approach for a particular task.


  1. Flexibility and Scalability: Zero-shot learning allows the model to classify data into new classes without requiring specific training examples for those classes. This flexibility makes extending the model to handle new classes easily without retraining.
  2. Reduced Data Annotation: Since zero-shot learning doesn’t require labelled examples for all classes, it can significantly reduce the need for extensive data annotation, which can be expensive and time-consuming in traditional supervised learning.
  3. Generalization to Unseen Classes: The model can generalize to classify data not seen during training as long as relevant semantic embeddings or descriptions are provided. This makes zero-shot learning useful for applications where new classes emerge frequently.
  4. Cross-Domain Transfer: Zero-shot learning can effectively transfer knowledge from one domain to another, leveraging pre-trained models’ semantic understanding to classify data in a related field with minimal adaptation.
  5. Adaptability to Multi-Label Classification: Zero-shot learning can handle multi-label classification naturally. It can assign multiple labels to an input instance if the class descriptions allow that.


  1. Performance Dependence on Class Descriptions: The quality and relevance of the provided class descriptions significantly impact zero-shot learning’s performance. The model might struggle to classify instances if the descriptions are inadequate or ambiguous correctly.
  2. Limited Performance on Dissimilar Classes: Zero-shot learning might not perform as well in classes that are very different from those seen during pre-training. The model’s performance degrades for classes that lack semantic similarity with the training data.
  3. Overfitting Risk: If the class descriptions are too specific or similar to the training data, the model might inadvertently overfit the given descriptions, leading to poorer generalization on unseen data.
  4. Sensitivity to Input Variations: Zero-shot learning can be sensitive to slight variations in the input, as the model heavily relies on semantic embeddings and context. Small changes in the input text might lead to different predictions.
  5. Evaluation Challenges: Measuring the performance of zero-shot learning can be challenging, as standard evaluation metrics used in supervised learning might not directly apply. Evaluating truly unseen classes can be complex and might require the careful design of evaluation protocols.

In summary, zero-shot learning offers great potential for flexible and scalable classification tasks, mainly when dealing with the frequent emergence of new classes. However, it requires careful consideration of class descriptions and might have limitations when dealing with dissimilar or ambiguous descriptions. Combining zero-shot learning with techniques like few-shot learning or fine-tuning can help mitigate these limitations and improve overall performance.

Are there any potential drawbacks or ethical concerns with zero-shot classification in content moderation systems?

Simply put: yes. Potential drawbacks and ethical concerns are associated with zero-shot classification in content moderation systems. While zero-shot classification can offer flexibility and scalability in handling new classes of content, it also introduces some challenges that need to be carefully addressed:

  1. Bias and Fairness: Zero-shot classification models heavily rely on the provided class descriptions. The model’s classification decisions can also be biased if the descriptions are biased or reflect societal prejudices. This could lead to unfair content moderation practices and unequal treatment of users based on their background, race, gender, or other sensitive attributes.
  2. Misclassification: Zero-shot classification might not always perform as accurately as supervised learning when dealing with unseen classes. There is a risk of misclassifying content, leading to false positives or negatives in moderation. Misclassifications can result in censoring legitimate content or allowing harmful content to slip through the moderation system.
  3. Ambiguity and Context: Zero-shot classification relies on the semantic understanding of class descriptions and context. If the class descriptions are ambiguous or lack sufficient context, the model might make incorrect or inconsistent classification decisions.
  4. Evading Detection: Adversaries might exploit weaknesses in zero-shot classification models to create content that evades moderation. By carefully crafting content to appear similar to benign classes, malicious content could potentially avoid detection.
  5. Content Subjectivity: Content moderation often involves subjective judgments about what is acceptable and what is not. The class descriptions used for zero-shot classification might not capture the complexities of content moderation policies, leading to disagreements between the model’s decisions and human moderators’ intentions.
  6. Privacy Concerns: In some content moderation systems, zero-shot classification might involve analyzing user-generated content, which can raise privacy concerns. Sensitive user information could inadvertently be used in class descriptions or be inferred from content classifications.

To address these concerns, content moderation systems using zero-shot classification need to be developed with care and include appropriate safeguards and mitigations:

  • Regular Model Updates: Continuously update and fine-tune the zero-shot classification model to address bias and improve classification accuracy. Monitor its performance and adjust class descriptions as needed.
  • Human-in-the-Loop: Incorporate human moderation to review flagged content and ensure the model’s decisions align with content moderation policies.
  • Transparent and Explainable AI: Make the content moderation process transparent and explain how their content is moderated to users. Transparency helps build trust with users.
  • User Feedback Mechanism: Allow users to provide feedback on moderation decisions, allowing them to contest incorrect classifications.
  • Privacy Protections: Handle user data and content with strict privacy protection to prevent unauthorized access or misuse.

Overall, while zero-shot classification can benefit content moderation systems, developers must be cautious about potential ethical challenges and work towards building responsible and fair AI systems.

Alternatives to zero-shot learning

Several alternative approaches to zero-shot learning exist for classification tasks. These methods vary in their complexity, data requirements, and performance. Some common alternatives include:

  1. Supervised Learning: A model is trained on a labelled dataset with examples for each class it needs to classify. This is the traditional approach to classification and is highly effective when a sufficient amount of labelled training data is available for all classes.
  2. Few-Shot Learning: Few-shot learning lies between zero-shot and fully supervised learning. It aims to classify data with only a few examples for each class. This approach is advantageous when labelled data is scarce for certain classes but available for others.
  3. Semi-Supervised Learning: Semi-supervised learning combines labelled and unlabeled data during training. It can leverage labelled examples for some classes and unlabeled data to improve classification performance.
  4. Transfer Learning: Transfer learning involves pre-training a model on a large dataset and then fine-tuning it on a smaller labelled dataset specific to the target task. This approach can be practical when the pre-trained model captures relevant features useful for the classification task.
  5. Multi-Task Learning: A single model is trained to perform multiple related tasks simultaneously in multi-task learning. It can help improve classification performance by leveraging knowledge from other related tasks.
  6. Active Learning: Active learning is an iterative approach where the model actively selects the most informative instances for labelling. This reduces the need for large amounts of labelled data and can improve classification performance with a smaller labelled dataset.
  7. Ensemble Methods: Ensemble methods combine predictions from multiple models to obtain more accurate and robust classifications. They can be used to improve classification performance when individual models might struggle to handle specific classes.
  8. Domain Adaptation: Domain adaptation aims to transfer knowledge from a source domain with labelled data to a target domain with different characteristics but lacks labelled data. It can be helpful when the target domain has another distribution from the source domain.
  9. Meta-Learning: Meta-learning, also known as “learning to learn,” trains a model to learn how to adapt quickly to new tasks with limited data. It can help handle new classes with only a few examples.

The choice of the most appropriate alternative depends on factors such as the availability of labelled data, the similarity between training and test distributions, and the task’s specific requirements. Each method has strengths and limitations, and combining different techniques might be necessary to achieve the best results for a particular classification task.


Classification tasks in machine learning and natural language processing have seen various advancements, and zero-shot learning has emerged as an intriguing approach to tackle some of the challenges associated with traditional supervised learning. Zero-shot learning allows models to classify data into new and unseen classes without specific training examples, offering flexibility, scalability, and adaptability to changing or expanding class sets.

The critical advantage of zero-shot learning lies in its ability to leverage semantic embeddings and class descriptions to generalize to unseen classes. This flexibility makes it useful for tasks with dynamic and evolving class sets, where the traditional supervised learning approach might be cumbersome.

However, zero-shot learning is not without its limitations. Its performance heavily depends on the quality and relevance of the class descriptions, and it may struggle with classes dissimilar to the training data. Additionally, evaluating zero-shot learning models can be challenging due to the lack of standard evaluation protocols.

As a result, practitioners need to design and fine-tune their zero-shot learning models carefully, considering the nature of the task, the quality of class descriptions, and any potential overfitting risks.

While zero-shot learning offers exciting possibilities, it is not a one-size-fits-all solution. Depending on the availability of labelled data and the specific requirements of the classification task, alternative approaches like supervised learning, few-shot learning, transfer learning, and domain adaptation might be more suitable and yield better performance.

Combining different approaches, including zero-shot learning, can create more robust and accurate classification models. As research continues, we will likely see further advancements and refinements to these approaches, leading to even more effective and efficient classification task classification methods.

About the Author

Neri Van Otten

Neri Van Otten

Neri Van Otten is the founder of Spot Intelligence, a machine learning engineer with over 12 years of experience specialising in Natural Language Processing (NLP) and deep learning innovation. Dedicated to making your projects succeed.

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1 Comment

  1. Daniel Scalioni Carvalho

    I only want to say thanks and congrats you to share all this basic information.
    This certainly guides me to a better path on my project.


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