Prompt Template

A lightweight, zero-dependency Python library for managing LLM prompt templates. Built on the design principles of string.Template but with enhanced features specifically designed for LLM prompt engineering.

Features

• Strong template validation with detailed error messages
• Support for nested braces and complex JSON structures
• Automatic value serialization for common Python types
• Default value support with deep copying for mutable types
• Incremental template population with value inheritance
• Type hints for enhanced IDE support
• Zero dependencies - just pure Python
• 100% test coverage

Installation

pip install prompt-template

Usage

The library is intentionally very simple to use. The idea is to keep it simple, have validation and serialization builtin, and make debugging simple.

Simple Templates

from prompt_template import PromptTemplate

# Create a template
template = PromptTemplate("Hello ${name}! Welcome to ${location}.")

# Render with values
result = template.to_string(name="Alice", location="Wonderland")
print(result)  # Hello Alice! Welcome to Wonderland.

Default Values

# Set default values that can be overridden later
template = PromptTemplate("Hello ${name}! Your settings are: ${settings}")

# Set default values - they're safely deep copied
template.set_default(
    name="Guest",
    settings={"theme": "light", "language": "en"}
)

# Use with defaults
print(template.to_string())
# Hello Guest! Your settings are: {"theme": "light", "language": "en"}

# Override specific values
print(template.to_string(name="Alice"))
# Hello Alice! Your settings are: {"theme": "light", "language": "en"}

# Override everything
print(template.to_string(
    name="Bob",
    settings={"theme": "dark", "language": "fr"}
))
# Hello Bob! Your settings are: {"theme": "dark", "language": "fr"}

Named Templates

Adding a name to your template enhances error messages with context. Templates with the same name and content are considered equal:

# Create named templates
template1 = PromptTemplate(
    name="user_greeting",
    template="Hello ${name}! Welcome to ${location}."
)

# Templates can be compared and used as dictionary keys
template2 = PromptTemplate(
    name="user_greeting",
    template="Hello ${name}! Welcome to ${location}."
)
assert template1 == template2

# Templates have a readable string representation
print(template1)  # PromptTemplate [user_greeting]:\n\nHello ${name}! Welcome to ${location}.

Complex JSON Templates

The library handles nested structures elegantly:

template = PromptTemplate("""
{
    "user": {
        "name": "${username}",
        "role": "${role}"
    },
    "settings": {
        "theme": "${theme}",
        "notifications": ${notifications},
        "preferences": ${preferences}
    }
}
""")

# Values are automatically serialized
result = template.to_string(
    username="john_doe",
    role="admin",
    theme="dark",
    notifications={"email": True, "push": False},
    preferences=["daily_digest", "weekly_report"]
)

Template Methods

The library provides two main methods for populating templates:

# to_string(): Validates inputs, uses defaults, and serializes values
template = PromptTemplate("Hello ${name}!")
template.set_default(name="Guest")
result = template.to_string()  # Uses default
result = template.to_string(name="Alice")  # Overrides default

# substitute(): Lower-level method for partial population
base = PromptTemplate("${greeting} ${name}!")
partial = base.substitute(greeting="Hello")  # Returns a string
final = PromptTemplate(partial).to_string(name="Alice")

Nested Templates

Templates can be nested within other templates:

# Create nested templates
inner = PromptTemplate("My name is ${name}")
inner.set_default(name="Alice")

outer = PromptTemplate("""
User message:
${message}
""")

# Combine templates
result = outer.to_string(message=inner.to_string())
print(result)
# User message:
# My name is Alice

Type Handling and Automatic Serialization

The library automatically handles various Python types:

from uuid import UUID
from decimal import Decimal
from datetime import datetime

template = PromptTemplate("""
{
    "id": "${id}",
    "amount": "${amount}",
    "metadata": ${metadata}
}
""")

result = template.to_string(
    id=UUID("550e8400-e29b-41d4-a716-446655440000"),
    amount=Decimal("45.67"),
    metadata={
        "timestamp": datetime.now(),
        "values": [1, 2, 3]
    }
)

Custom Serialization

You can override or extend serialization by subclassing PromptTemplate and implementing the serializer method:

from typing import Any
from datetime import datetime
from decimal import Decimal
from prompt_template import PromptTemplate as BasePromptTemplate
import orjson


class PromptTemplate(BasePromptTemplate):
    @staticmethod
    def serializer(value: Any) -> str:
        """Custom serializer with orjson for better performance.
        """
        try:
            return orjson.dumps(value).decode('utf-8')
        except Exception:
            return super().serializer(value)

Error Handling

The library provides specific errors classes for error handling:

Missing Values

from prompt_template import MissingTemplateValuesError

template = PromptTemplate("Hello ${name}!")
try:
    template.to_string()  # No values provided
except MissingTemplateValuesError as e:
    print(f"Missing values: {e.missing_values}")  # {'name'}

Invalid Keys

from prompt_template import InvalidTemplateKeysError

template = PromptTemplate("Hello ${name}!")
try:
    template.to_string(name="World", invalid_key="value")
except InvalidTemplateKeysError as e:
    print(f"Invalid keys: {e.invalid_keys}")  # ['invalid_key']
    print(f"Valid keys: {e.valid_keys}")  # {'name'}

Serialization Errors

from prompt_template import TemplateSerializationError

template = PromptTemplate("Value: ${value}")
try:
    template.to_string(value=object())  # Non-serializable object
except TemplateSerializationError as e:
    print(f"Failed to serialize key '{e.key}': {e.original_error}")

License

MIT License