Understand Cost and Usage of Operations

When using LLMs for evaluation and test set generation, cost will be an important factor. Ragas provides you some tools to help you with that.

Understanding TokenUsageParser

By default Ragas does not calculate the usage of tokens for evaluate(). This is because langchain's LLMs do not always return information about token usage in a uniform way. So in order to get the usage data, we have to implement a TokenUsageParser.

A TokenUsageParser is function that parses the LLMResult or ChatResult from langchain models generate_prompt() function and outputs TokenUsage which Ragas expects.

For an example here is one that will parse OpenAI by using a parser we have defined.

from langchain_openai.chat_models import ChatOpenAI
from langchain_core.prompt_values import StringPromptValue

gpt4o = ChatOpenAI(model="gpt-4o")
p = StringPromptValue(text="hai there")
llm_result = gpt4o.generate_prompt([p])

# lets import a parser for OpenAI
from ragas.cost import get_token_usage_for_openai

get_token_usage_for_openai(llm_result)

TokenUsage(input_tokens=9, output_tokens=9, model='')

You can define your own or import parsers if they are defined. If you would like to suggest parser for LLM providers or contribute your own ones please check out this issue 🙂.

You can use it for evaluations as so. Using example from get started here.

from datasets import load_dataset
from ragas.metrics import (
    answer_relevancy,
    faithfulness,
    context_recall,
    context_precision,
)

amnesty_qa = load_dataset("explodinggradients/amnesty_qa", "english_v2")
amnesty_qa

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DatasetDict({
    eval: Dataset({
        features: ['question', 'ground_truth', 'answer', 'contexts'],
        num_rows: 20
    })
})

from ragas import evaluate
from ragas.cost import get_token_usage_for_openai

result = evaluate(
    amnesty_qa["eval"],
    metrics=[
        context_precision,
        faithfulness,
        answer_relevancy,
        context_recall,
    ],
    llm=gpt4o,
    token_usage_parser=get_token_usage_for_openai,
)

Evaluating:   0%|          | 0/80 [00:00<?, ?it/s]

result.total_tokens()

TokenUsage(input_tokens=116765, output_tokens=39031, model='')

You can compute the cost for each run by passing in the cost per token to Result.total_cost() function.

In this case GPT-4o costs $5 for 1M input tokens and $15 for 1M output tokens.

result.total_cost(cost_per_input_token=5 / 1e6, cost_per_output_token=15 / 1e6)

1.1692900000000002