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Introduction to Evaluations

Weave is a toolkit for developing AI-powered applications.

You can use Weave to:

• Log and debug language model inputs, outputs, and traces.
• Build rigorous, apples-to-apples evaluations for language model use cases.
• Organize all the information generated across the LLM workflow, from experimentation to evaluations to production.

This notebook demonstrates how to evaluate a model or function using Weave’s Evaluation API. Evaluation is a core concept in Weave that helps you measure and iterate on your application by running it against a dataset of examples and scoring the outputs using custom-defined functions. You'll define a simple model, create a labeled dataset, track scoring functions with @weave.op, and run an evaluation that automatically tracks results in the Weave UI. This forms the foundation for more advanced workflows like LLM fine-tuning, regression testing, or model comparison.

To get started, complete the prerequisites. Then, define a Weave Model with a predict method, create a labeled dataset and scoring function, and run an evaluation using weave.Evaluation.evaluate().

🔑 Prerequisites

Before you can run a Weave evaluation, complete the following prerequisites.

1. Install the W&B Weave SDK and log in with your API key.
2. Install the OpenAI SDK and log in with your API key.
3. Initialize your W&B project.

# Install dependancies and imports
!pip install wandb weave openai -q

import os
from getpass import getpass

from openai import OpenAI
from pydantic import BaseModel

import weave

# 🔑 Setup your API keys
# Running this cell will prompt you for your API key with `getpass` and will not echo to the terminal.
#####
print("---")
print(
    "You can find your Weights and Biases API key here: https://wandb.ai/settings#api"
)
os.environ["WANDB_API_KEY"] = getpass("Enter your Weights and Biases API key: ")
print("---")
print("You can generate your OpenAI API key here: https://platform.openai.com/api-keys")
os.environ["OPENAI_API_KEY"] = getpass("Enter your OpenAI API key: ")
print("---")
#####

# 🏠 Enter your W&B project name
weave_client = weave.init("MY_PROJECT_NAME")  # 🐝 Your W&B project name

🐝 Run your first evaluation

The following code sample shows how to evaluate an LLM using Weave’s Model and Evaluation APIs. First, define a Weave model by subclassing weave.Model, specifying the model name and prompt format, and tracking a predict method with @weave.op. The predict method sends a prompt to OpenAI and parses the response into a structured output using a Pydantic schema (FruitExtract). Then, create a small evaluation dataset consisting of input sentences and expected targets. Next, define a custom scoring function (also tracked using @weave.op) that compares the model’s output to the target label. Finally, wrap everything in a weave.Evaluation, specifying your dataset and scorers, and call evaluate() to run the evaluation pipeline asynchronously.

# 1. Construct a Weave model
class FruitExtract(BaseModel):
    fruit: str
    color: str
    flavor: str


class ExtractFruitsModel(weave.Model):
    model_name: str
    prompt_template: str

    @weave.op()
    def predict(self, sentence: str) -> dict:
        client = OpenAI()

        response = client.beta.chat.completions.parse(
            model=self.model_name,
            messages=[
                {
                    "role": "user",
                    "content": self.prompt_template.format(sentence=sentence),
                }
            ],
            response_format=FruitExtract,
        )
        result = response.choices[0].message.parsed
        return result


model = ExtractFruitsModel(
    name="gpt4o",
    model_name="gpt-4o",
    prompt_template='Extract fields ("fruit": <str>, "color": <str>, "flavor": <str>) as json, from the following text : {sentence}',
)

# 2. Collect some samples
sentences = [
    "There are many fruits that were found on the recently discovered planet Goocrux. There are neoskizzles that grow there, which are purple and taste like candy.",
    "Pounits are a bright green color and are more savory than sweet.",
    "Finally, there are fruits called glowls, which have a very sour and bitter taste which is acidic and caustic, and a pale orange tinge to them.",
]
labels = [
    {"fruit": "neoskizzles", "color": "purple", "flavor": "candy"},
    {"fruit": "pounits", "color": "green", "flavor": "savory"},
    {"fruit": "glowls", "color": "orange", "flavor": "sour, bitter"},
]
examples = [
    {"id": "0", "sentence": sentences[0], "target": labels[0]},
    {"id": "1", "sentence": sentences[1], "target": labels[1]},
    {"id": "2", "sentence": sentences[2], "target": labels[2]},
]


# 3. Define a scoring function for your evaluation
@weave.op()
def fruit_name_score(target: dict, output: FruitExtract) -> dict:
    target_flavors = [f.strip().lower() for f in target["flavor"].split(",")]
    output_flavors = [f.strip().lower() for f in output.flavor.split(",")]
    # Check if any target flavor is present in the output flavors
    matches = any(tf in of for tf in target_flavors for of in output_flavors)
    return {"correct": matches}


# 4. Run your evaluation
evaluation = weave.Evaluation(
    name="fruit_eval",
    dataset=examples,
    scorers=[fruit_name_score],
)
await evaluation.evaluate(model)

🚀 Looking for more examples?

• Learn how to build an evlauation pipeline end-to-end.
• Learn how to evaluate a RAG application by building.