Few-Shot Prompting Techniques

Large Language Models (LLMs) possess impressive capabilities, but they often perform better when shown how to complete a task rather than just being told what to do. This is where few-shot prompting comes into play. Few-shot prompting is a technique that includes several examples (shots) of the task being performed directly within the prompt itself. This differs from zero-shot prompting, an approach that provides only the instruction without any examples.

Think of it as giving the model a mini-tutorial right before asking it to solve a new problem. These examples act as demonstrations, guiding the model towards the desired output format, style, or reasoning process. This technique is particularly useful for tasks where the desired output structure is specific or when the task itself requires a pattern that isn't immediately obvious from the instruction alone.

Zero-Shot vs. One-Shot vs. Few-Shot

Let's clarify the distinction:

• Zero-Shot: The prompt contains only the instruction and the input. The model relies entirely on its pre-trained knowledge.
  • Example: Translate to French: Hello world
• One-Shot: The prompt includes one example before the actual input. This gives the model a single demonstration.
  • Example:

    Translate to French:
    sea otter => loutre de mer
    Hello world =>
    

• Few-Shot: The prompt provides multiple examples (typically 2 to 5, but sometimes more). This offers stronger guidance and helps the model generalize the pattern better.
  • Example:

    Translate to French:
    sea otter => loutre de mer
    cheese => fromage
    blue sky => ciel bleu
    Hello world =>
    

Why Use Few-Shot Prompting?

Providing multiple examples offers several advantages:

1. Improved Accuracy: For many tasks, especially those involving specific formats or classifications, few-shot prompts significantly outperform zero-shot prompts. The examples constrain the model's possible outputs.
2. Format Control: If you need the output in a precise format (like JSON, specific capitalization, or a certain sentence structure), few-shot examples are an effective way to demonstrate it.
3. Task Clarification: Complex instructions can sometimes be ambiguous. Examples provide concrete illustrations of what's expected.
4. Handling Novel Tasks: When asking the model to perform a task it might not have explicitly seen during training (but which involves patterns it does understand), few-shot learning helps it adapt quickly.

Constructing Few-Shot Prompts in Python

In Python, you can construct few-shot prompts using simple string formatting or more structured approaches with libraries like LangChain.

Basic String Formatting

You can use f-strings or str.format() to build the prompt dynamically.

# Examples of input/output pairs
examples = [
    {"input": "A friendly water mammal.", "output": "sea otter"},
    {"input": "A dairy product made from milk.", "output": "cheese"},
    {"input": "The color of the atmosphere on a clear day.", "output": "blue sky"}
]

# The new input we want the model to process
new_input = "A large, gray animal with a trunk."

# Construct the prompt string
prompt_parts = ["Identify the object from the description.\n"]
for example in examples:
    prompt_parts.append(f"Description: {example['input']}")
    prompt_parts.append(f"Object: {example['output']}\n") # Add a newline for separation

# Add the final input
prompt_parts.append(f"Description: {new_input}")
prompt_parts.append("Object:") # Prompt the model for the final output

final_prompt = "\n".join(prompt_parts)

print(final_prompt)

# Expected Output:
# Identify the object from the description.
#
# Description: A friendly water mammal.
# Object: sea otter
#
# Description: A dairy product made from milk.
# Object: cheese
#
# Description: The color of the atmosphere on a clear day.
# Object: blue sky
#
# Description: A large, gray animal with a trunk.
# Object:

This final_prompt string would then be sent to the LLM API. The model sees the pattern in the examples and is likely to output "elephant".

Using LangChain for Structured Prompts

Frameworks like LangChain provide ways to manage prompts, especially few-shot prompts. You can use classes like FewShotPromptTemplate. (We explored PromptTemplate in Chapter 4; FewShotPromptTemplate builds upon that).

from langchain_core.prompts import FewShotPromptTemplate, PromptTemplate

# Define the examples
examples = [
    {"input": "happy", "output": "sad"},
    {"input": "tall", "output": "short"},
]

# Define the template for how each example should be formatted
example_prompt = PromptTemplate.from_template("Input: {input}\nOutput: {output}")

# Define the overall few-shot prompt template
few_shot_prompt = FewShotPromptTemplate(
    examples=examples,
    example_prompt=example_prompt,
    prefix="Give the antonym of the input word.",
    suffix="Input: {user_input}\nOutput:", # {user_input} is the variable for the final input
    input_variables=["user_input"], # Specifies the variable name for the final input
    example_separator="\n\n" # Separator between examples
)

# Format the prompt for a new input
final_prompt = few_shot_prompt.format(user_input="hot")

print(final_prompt)

# Expected Output:
# Give the antonym of the input word.
#
# Input: happy
# Output: sad
#
# Input: tall
# Output: short
#
# Input: hot
# Output:

Using FewShotPromptTemplate makes managing examples, formatting, and the overall prompt structure cleaner, especially as prompts become more complex.

Selecting Effective Examples

The quality of your examples is significant. Poor examples can confuse the model or lead it to replicate errors. Keep these points in mind:

• Relevance: Examples should closely match the actual task you want the model to perform.
• Clarity: The input-output relationship in each example should be clear and unambiguous.
• Consistency: Ensure the format and style of the output in your examples are consistent with what you desire for new inputs.
• Diversity (if applicable): If the task involves variations, try to include examples covering different aspects or edge cases. However, don't make the examples contradict each other.
• Accuracy: Double-check that your examples are correct. The model learns from them, including any mistakes.

When to Use Few-Shot Prompting

Few-shot prompting is particularly effective in scenarios such as:

• Text Classification: Guiding the model to use specific category labels.
• Format Conversion: Transforming data from one structure to another (e.g., text to JSON).
• Code Generation: Providing examples of code snippets for a specific pattern.
• Stylistic Control: Ensuring the response adheres to a particular tone or style (e.g., formal, informal, poetic).
• Complex Instructions: Breaking down a task through demonstration when a simple instruction is insufficient.

Limitations and Challenges

While powerful, few-shot prompting has aspects to consider:

• Context Window Limits: Every example adds tokens to your prompt. LLMs have maximum context window sizes (the total number of tokens they can process at once). Too many examples can exceed this limit.
• Cost: LLM API calls are often priced based on the number of input and output tokens. Longer prompts with many examples will increase the cost per API call.
• Example Selection Effort: Finding or creating good examples requires careful thought and potentially some experimentation.

Few-shot prompting is a fundamental technique in practical prompt engineering. By providing concrete examples within the prompt, you significantly enhance your ability to guide LLM behavior and achieve more reliable, accurate, and correctly formatted results, especially when implementing these prompts programmatically using Python.