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The format Method

Functions created using magentic decorators expose a format method that accepts the same parameters as the function itself but returns the completed prompt that will be sent to the model. For @prompt this method returns a string, and for @chatprompt it returns a list of Message objects. The format method can be used to test that the final prompt created by a magentic function is formatted as expected.

from magentic import prompt

@prompt("Write a short poem about {topic}.")
def create_poem(topic: str) -> str: ...

# 'Write a short poem about fruit.'

Classes for Formatting

By default, when a list is used in a prompt template string it is formatted using its Python representation.

from magentic import prompt
from magentic.formatting import BulletedList

@prompt("Continue the list:\n{items}")
def get_next_items(items: list[str]) -> list[str]: ...

items = ["apple", "banana", "cherry"]
# Continue the list:
# ['apple', 'banana', 'cherry']

However, the LLM might respond better to a prompt in which the list is formatted more clearly or the items are numbered. The BulletedList, NumberedList, BulletedDict and NumberedDict classes are provided to enable this.

For example, to modify the above prompt to contain a numbered list of the items, the NumberedList class can be used. This behaves exactly like a regular Python list except for how it appears when inserted into a formatted string. This class can also be used as the type annotation for items parameter to ensure that this prompt always contains a numbered list.

from magentic import prompt
from magentic.formatting import NumberedList

@prompt("Continue the list:\n{items}")
def get_next_items(items: NumberedList[str]) -> list[str]: ...

items = NumberedList(["apple", "banana", "cherry"])
# Continue the list:
# 1. apple
# 2. banana
# 3. cherry

Custom Formatting

When objects are inserted into formatted strings in Python, the __format__ method is called. By defining or modifying this method you can control how an object is converted to a string in the prompt. If you own the class you can modify the __format__ method directly. Otherwise for third-party classes you will need to create a subcless.

Here's an example of how to represent a dictionary as a bulleted list.

from typing import TypeVar

from magentic import prompt

K = TypeVar("K")
V = TypeVar("V")

class BulletedDict(dict[K, V]):
    def __format__(self, format_spec: str) -> str:
        # Here, you could use 'format_spec' to customize the formatting further if needed
        return "\n".join(f"- {key}: {value}" for key, value in self.items())

@prompt("Identify the odd one out:\n{items}")
def find_odd_one_out(items: BulletedDict[str, str]) -> str: ...

items = BulletedDict({"sky": "blue", "grass": "green", "sun": "purple"})
# Identify the odd one out:
# - sky: blue
# - grass: green
# - sun: purple