Custom String Evaluator

You can make your own custom string evaluators by inheriting from the StringEvaluator class and implementing the _evaluate_strings (and _aevaluate_strings for async support) methods.

In this example, you will create a perplexity evaluator using the HuggingFace evaluate library. Perplexity is a measure of how well the generated text would be predicted by the model used to compute the metric.

%pip install --upgrade --quiet  evaluate > /dev/null

from typing import Any, Optional

from evaluate import load
from langchain.evaluation import StringEvaluator


class PerplexityEvaluator(StringEvaluator):
    """Evaluate the perplexity of a predicted string."""

    def __init__(self, model_id: str = "gpt2"):
        self.model_id = model_id
        self.metric_fn = load(
            "perplexity", module_type="metric", model_id=self.model_id, pad_token=0
        )

    def _evaluate_strings(
        self,
        *,
        prediction: str,
        reference: Optional[str] = None,
        input: Optional[str] = None,
        **kwargs: Any,
    ) -> dict:
        results = self.metric_fn.compute(
            predictions=[prediction], model_id=self.model_id
        )
        ppl = results["perplexities"][0]
        return {"score": ppl}

evaluator = PerplexityEvaluator()

evaluator.evaluate_strings(prediction="The rains in Spain fall mainly on the plain.")

Using pad_token, but it is not set yet.
``````output
huggingface/tokenizers: The current process just got forked, after parallelism has already been used. Disabling parallelism to avoid deadlocks...
To disable this warning, you can either:
    - Avoid using `tokenizers` before the fork if possible
    - Explicitly set the environment variable TOKENIZERS_PARALLELISM=(true | false)

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{'score': 190.3675537109375}

# The perplexity is much higher since LangChain was introduced after 'gpt-2' was released and because it is never used in the following context.
evaluator.evaluate_strings(prediction="The rains in Spain fall mainly on LangChain.")

Using pad_token, but it is not set yet.

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{'score': 1982.0709228515625}