DSPy Signature Designer

Goal

Design clear, type-safe signatures that define what your DSPy modules should do.

When to Use

• Defining new DSPy modules
• Need structured/validated outputs
• Complex input/output relationships
• Multi-field responses

Inputs

Outputs

Workflow

Inline Signatures (Simple)

import dspy

# Basic
qa = dspy.Predict("question -> answer")

# With types
classify = dspy.Predict("sentence -> sentiment: bool")

# Multiple fields
rag = dspy.ChainOfThought("context: list[str], question: str -> answer: str")

Class-based Signatures (Complex)

from typing import Literal, Optional
import dspy

class EmotionClassifier(dspy.Signature):
    """Classify the emotion expressed in the text."""
    
    text: str = dspy.InputField(desc="The text to analyze")
    emotion: Literal['joy', 'sadness', 'anger', 'fear', 'surprise'] = dspy.OutputField()
    confidence: float = dspy.OutputField(desc="Confidence score 0-1")

Type Hints Reference

from typing import Literal, Optional, List
from pydantic import BaseModel

# Basic types
field: str = dspy.InputField()
field: int = dspy.OutputField()
field: float = dspy.OutputField()
field: bool = dspy.OutputField()

# Collections
field: list[str] = dspy.InputField()
field: List[int] = dspy.OutputField()

# Optional
field: Optional[str] = dspy.OutputField()

# Constrained
field: Literal['a', 'b', 'c'] = dspy.OutputField()

# Pydantic models
class Person(BaseModel):
    name: str
    age: int

field: Person = dspy.OutputField()

Production Examples

Summarization

class Summarize(dspy.Signature):
    """Summarize the document into key points."""
    
    document: str = dspy.InputField(desc="Full document text")
    max_points: int = dspy.InputField(desc="Maximum bullet points", default=5)
    
    summary: list[str] = dspy.OutputField(desc="Key points as bullet list")
    word_count: int = dspy.OutputField(desc="Total words in summary")

Entity Extraction

from pydantic import BaseModel
from typing import List

class Entity(BaseModel):
    text: str
    type: str
    start: int
    end: int

class ExtractEntities(dspy.Signature):
    """Extract named entities from text."""
    
    text: str = dspy.InputField()
    entity_types: list[str] = dspy.InputField(
        desc="Types to extract: PERSON, ORG, LOC, DATE",
        default=["PERSON", "ORG", "LOC"]
    )
    
    entities: List[Entity] = dspy.OutputField()

Multi-Label Classification

class MultiLabelClassify(dspy.Signature):
    """Classify text into multiple categories."""
    
    text: str = dspy.InputField()
    
    categories: list[str] = dspy.OutputField(
        desc="Applicable categories from: tech, business, sports, entertainment"
    )
    primary_category: str = dspy.OutputField(desc="Most relevant category")
    reasoning: str = dspy.OutputField(desc="Explanation for classification")

RAG with Confidence

class GroundedAnswer(dspy.Signature):
    """Answer questions using retrieved context with confidence."""
    
    context: list[str] = dspy.InputField(desc="Retrieved passages")
    question: str = dspy.InputField()
    
    answer: str = dspy.OutputField(desc="Factual answer from context")
    confidence: Literal['high', 'medium', 'low'] = dspy.OutputField(
        desc="Confidence based on context support"
    )
    source_passage: int = dspy.OutputField(
        desc="Index of most relevant passage (0-based)"
    )

Complete Module with Signature

import dspy
from typing import Literal, Optional
import logging

logger = logging.getLogger(__name__)

class AnalyzeSentiment(dspy.Signature):
    """Analyze sentiment with detailed breakdown."""
    
    text: str = dspy.InputField(desc="Text to analyze")
    
    sentiment: Literal['positive', 'negative', 'neutral', 'mixed'] = dspy.OutputField()
    score: float = dspy.OutputField(desc="Sentiment score from -1 to 1")
    aspects: list[str] = dspy.OutputField(desc="Key aspects mentioned")
    reasoning: str = dspy.OutputField(desc="Explanation of sentiment")

class SentimentAnalyzer(dspy.Module):
    def __init__(self):
        self.analyze = dspy.ChainOfThought(AnalyzeSentiment)
    
    def forward(self, text: str):
        try:
            result = self.analyze(text=text)
            
            # Validate score range
            if hasattr(result, 'score'):
                result.score = max(-1, min(1, float(result.score)))
            
            return result
            
        except Exception as e:
            logger.error(f"Analysis failed: {e}")
            return dspy.Prediction(
                sentiment='neutral',
                score=0.0,
                aspects=[],
                reasoning="Analysis failed"
            )

# Usage
analyzer = SentimentAnalyzer()
result = analyzer(text="The product quality is great but shipping was slow.")
print(f"Sentiment: {result.sentiment} ({result.score})")
print(f"Aspects: {result.aspects}")

Best Practices

1. Descriptive docstrings - The class docstring becomes the task instruction
2. Field descriptions - Guide the model with desc parameter
3. Constrain outputs - Use Literal for categorical outputs
4. Default values - Provide sensible defaults for optional inputs
5. Validate types - Pydantic models ensure structured output

Advanced Field Options

# Constraints (available in 3.2.1+)
class ConstrainedSignature(dspy.Signature):
    """Example with validation constraints."""

    text: str = dspy.InputField(
        min_length=5,
        max_length=100,
        desc="Input text between 5-100 chars"
    )
    number: int = dspy.InputField(
        gt=0,
        lt=10,
        desc="Number between 0 and 10"
    )
    score: float = dspy.OutputField(
        ge=0.0,
        le=1.0,
        desc="Score between 0 and 1"
    )
    count: int = dspy.OutputField(
        multiple_of=2,
        desc="Even number count"
    )

# Prefix and format
class FormattedSignature(dspy.Signature):
    """Example with custom prefix and format."""

    goal: str = dspy.InputField(prefix="Goal:")
    text: str = dspy.InputField(format=lambda x: x.upper())
    action: str = dspy.OutputField(prefix="Action:")

Limitations

• Complex nested types require Pydantic models
• Some LLMs struggle with strict type constraints
• Field descriptions and constraints add to prompt length
• Default values only work for InputField, not OutputField

Official Documentation

• DSPy Documentation: https://dspy.ai/
• DSPy GitHub: https://github.com/stanfordnlp/dspy
• Signatures API: https://dspy.ai/api/signatures/
• Signatures Guide: https://dspy.ai/learn/programming/signatures/