langchain/docs/extras/modules/evaluation/string/custom.ipynb

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    "# Custom String Evaluator\n",
    "\n",
    "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.\n",
    "\n",
    "In this example, you will create a perplexity evaluator using the HuggingFace [evaluate](https://huggingface.co/docs/evaluate/index) library.\n",
    "[Perplexity](https://en.wikipedia.org/wiki/Perplexity) is a measure of how well the generated text would be predicted by the model used to compute the metric."
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   "source": [
    "# %pip install evaluate > /dev/null"
   ]
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    "from typing import Any, Optional\n",
    "\n",
    "from langchain.evaluation import StringEvaluator\n",
    "from evaluate import load\n",
    "\n",
    "\n",
    "class PerplexityEvaluator(StringEvaluator):\n",
    "    \"\"\"Evaluate the perplexity of a predicted string.\"\"\"\n",
    "\n",
    "    def __init__(self, model_id: str = \"gpt2\"):\n",
    "        self.model_id = model_id\n",
    "        self.metric_fn = load(\n",
    "            \"perplexity\", module_type=\"metric\", model_id=self.model_id, pad_token=0\n",
    "        )\n",
    "\n",
    "    def _evaluate_strings(\n",
    "        self,\n",
    "        *,\n",
    "        prediction: str,\n",
    "        reference: Optional[str] = None,\n",
    "        input: Optional[str] = None,\n",
    "        **kwargs: Any,\n",
    "    ) -> dict:\n",
    "        results = self.metric_fn.compute(\n",
    "            predictions=[prediction], model_id=self.model_id\n",
    "        )\n",
    "        ppl = results[\"perplexities\"][0]\n",
    "        return {\"score\": ppl}"
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   "source": [
    "evaluator = PerplexityEvaluator()"
   ]
  },
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     "text": [
      "Using pad_token, but it is not set yet.\n"
     ]
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     "text": [
      "huggingface/tokenizers: The current process just got forked, after parallelism has already been used. Disabling parallelism to avoid deadlocks...\n",
      "To disable this warning, you can either:\n",
      "\t- Avoid using `tokenizers` before the fork if possible\n",
      "\t- Explicitly set the environment variable TOKENIZERS_PARALLELISM=(true | false)\n"
     ]
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       "{'score': 190.3675537109375}"
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   "source": [
    "evaluator.evaluate_strings(prediction=\"The rains in Spain fall mainly on the plain.\")"
   ]
  },
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      "Using pad_token, but it is not set yet.\n"
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       "{'score': 1982.0709228515625}"
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   "source": [
    "# The perplexity is much higher since LangChain was introduced after 'gpt-2' was released and because it is never used in the following context.\n",
    "evaluator.evaluate_strings(prediction=\"The rains in Spain fall mainly on LangChain.\")"
   ]
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