Custom pairwise evaluator

You can make your own pairwise string evaluators by inheriting from PairwiseStringEvaluator class and overwriting the _evaluate_string_pairs method (and the _aevaluate_string_pairs method if you want to use the evaluator asynchronously).

In this example, you will make a simple custom evaluator that just returns whether the first prediction has more whitespace tokenized 'words' than the second.

You can check out the reference docs for the PairwiseStringEvaluator interface for more info.

from typing import Any, Optional

from langchain.evaluation import PairwiseStringEvaluator


class LengthComparisonPairwiseEvaluator(PairwiseStringEvaluator):
    """
    Custom evaluator to compare two strings.
    """

    def _evaluate_string_pairs(
        self,
        *,
        prediction: str,
        prediction_b: str,
        reference: Optional[str] = None,
        input: Optional[str] = None,
        **kwargs: Any,
    ) -> dict:
        score = int(len(prediction.split()) > len(prediction_b.split()))
        return {"score": score}

API Reference:

• PairwiseStringEvaluator

evaluator = LengthComparisonPairwiseEvaluator()

evaluator.evaluate_string_pairs(
    prediction="The quick brown fox jumped over the lazy dog.",
    prediction_b="The quick brown fox jumped over the dog.",
)

{'score': 1}

LLM-Based Example

That example was simple to illustrate the API, but it wasn't very useful in practice. Below, use an LLM with some custom instructions to form a simple preference scorer similar to the built-in PairwiseStringEvalChain. We will use ChatAnthropic for the evaluator chain.

%pip install --upgrade --quiet  anthropic
# %env ANTHROPIC_API_KEY=YOUR_API_KEY

from typing import Any, Optional

from langchain.chains import LLMChain
from langchain.evaluation import PairwiseStringEvaluator
from langchain_community.chat_models import ChatAnthropic


class CustomPreferenceEvaluator(PairwiseStringEvaluator):
    """
    Custom evaluator to compare two strings using a custom LLMChain.
    """

    def __init__(self) -> None:
        llm = ChatAnthropic(model="claude-2", temperature=0)
        self.eval_chain = LLMChain.from_string(
            llm,
            """Which option is preferred? Do not take order into account. Evaluate based on accuracy and helpfulness. If neither is preferred, respond with C. Provide your reasoning, then finish with Preference: A/B/C

Input: How do I get the path of the parent directory in python 3.8?
Option A: You can use the following code:
```python
import os

os.path.dirname(os.path.dirname(os.path.abspath(__file__)))

API Reference:

• LLMChain
• PairwiseStringEvaluator
• ChatAnthropic

Option B: You can use the following code:

from pathlib import Path
Path(__file__).absolute().parent

Reasoning: Both options return the same result. However, since option B is more concise and easily understand, it is preferred. Preference: B

Which option is preferred? Do not take order into account. Evaluate based on accuracy and helpfulness. If neither is preferred, respond with C. Provide your reasoning, then finish with Preference: A/B/C Input: {input} Option A: {prediction} Option B: {prediction_b} Reasoning:""", )

@property
def requires_input(self) -> bool:
    return True

@property
def requires_reference(self) -> bool:
    return False

def _evaluate_string_pairs(
    self,
    *,
    prediction: str,
    prediction_b: str,
    reference: Optional[str] = None,
    input: Optional[str] = None,
    **kwargs: Any,
) -> dict:
    result = self.eval_chain(
        {
            "input": input,
            "prediction": prediction,
            "prediction_b": prediction_b,
            "stop": ["Which option is preferred?"],
        },
        **kwargs,
    )

    response_text = result["text"]
    reasoning, preference = response_text.split("Preference:", maxsplit=1)
    preference = preference.strip()
    score = 1.0 if preference == "A" else (0.0 if preference == "B" else None)
    return {"reasoning": reasoning.strip(), "value": preference, "score": score}

```python
evaluator = CustomPreferenceEvaluator()

evaluator.evaluate_string_pairs(
    input="How do I import from a relative directory?",
    prediction="use importlib! importlib.import_module('.my_package', '.')",
    prediction_b="from .sibling import foo",
)

{'reasoning': 'Option B is preferred over option A for importing from a relative directory, because it is more straightforward and concise.\n\nOption A uses the importlib module, which allows importing a module by specifying the full name as a string. While this works, it is less clear compared to option B.\n\nOption B directly imports from the relative path using dot notation, which clearly shows that it is a relative import. This is the recommended way to do relative imports in Python.\n\nIn summary, option B is more accurate and helpful as it uses the standard Python relative import syntax.',
 'value': 'B',
 'score': 0.0}

# Setting requires_input to return True adds additional validation to avoid returning a grade when insufficient data is provided to the chain.

try:
    evaluator.evaluate_string_pairs(
        prediction="use importlib! importlib.import_module('.my_package', '.')",
        prediction_b="from .sibling import foo",
    )
except ValueError as e:
    print(e)

CustomPreferenceEvaluator requires an input string.