fix: non llm based metrics (#1268)

1) rename metrics
2) delay import of optional dependencies

Author: shahules786

src/ragas/metrics/bleu_score.py renamed to src/ragas/metrics/_bleu_score.py

@@ -2,8 +2,6 @@
 from dataclasses import dataclass, field
 
 from langchain_core.callbacks import Callbacks
-from nltk.tokenize import word_tokenize
-from nltk.translate.bleu_score import corpus_bleu
 
 from ragas.dataset_schema import SingleTurnSample
 from ragas.metrics._faithfulness import HasSegmentMethod
@@ -21,7 +19,16 @@ class BleuScore(SingleTurnMetric):
     sentence_segmenter: t.Optional[HasSegmentMethod] = None
 
     def __post_init__(self):
+        try:
+            from nltk.tokenize import word_tokenize
+            from nltk.translate.bleu_score import corpus_bleu
+        except ImportError:
+            raise ImportError(
+                "nltk is required for bleu score. Please install it using `pip install nltk`"
+            )
         self.segmenter = get_segmenter()
+        self.word_tokenizer = word_tokenize
+        self.corpus_bleu = corpus_bleu
 
     def init(self, run_config: RunConfig):
         pass
@@ -32,9 +39,11 @@ async def _single_turn_ascore(
         reference_sentences = self.segmenter.segment(sample.reference)
         response_sentences = self.segmenter.segment(sample.response)
 
-        reference = [[word_tokenize(reference)] for reference in reference_sentences]
-        response = [word_tokenize(response) for response in response_sentences]
-        score = corpus_bleu(reference, response, weights=self.weights)
+        reference = [
+            [self.word_tokenizer(reference)] for reference in reference_sentences
+        ]
+        response = [self.word_tokenizer(response) for response in response_sentences]
+        score = self.corpus_bleu(reference, response, weights=self.weights)
         assert isinstance(score, float), "Expecting a float"
         return score
 

src/ragas/metrics/_string.py

@@ -3,7 +3,6 @@
 from enum import Enum
 
 from langchain_core.callbacks import Callbacks
-from rapidfuzz import distance
 
 from ragas.dataset_schema import SingleTurnSample
 from ragas.metrics.base import MetricType, SingleTurnMetric
@@ -16,13 +15,6 @@ class DistanceMeasure(Enum):
     JARO = "jaro"
 
 
-DISTANCE_MEASURE_MAP = {
-    DistanceMeasure.LEVENSHTEIN: distance.Levenshtein,
-    DistanceMeasure.HAMMING: distance.Hamming,
-    DistanceMeasure.JARO: distance.Jaro,
-}
-
-
 @dataclass
 class ExactMatch(SingleTurnMetric):
     name: str = "exact_match"  # type: ignore
@@ -42,6 +34,7 @@ async def _ascore(self, row: t.Dict, callbacks: Callbacks) -> float:
         return await self._single_turn_ascore(SingleTurnSample(**row), callbacks)
 
 
+@dataclass
 class StringPresent(SingleTurnMetric):
     name: str = "string_present"  # type: ignore
     _required_columns: t.Dict[MetricType, t.Set[str]] = field(
@@ -64,13 +57,28 @@ async def _ascore(self, row: t.Dict, callbacks: Callbacks) -> float:
         return await self._single_turn_ascore(SingleTurnSample(**row), callbacks)
 
 
-class StringDistance(SingleTurnMetric):
-    name: str = "string_distance"  # type: ignore
+@dataclass
+class NonLLMStringSimilarity(SingleTurnMetric):
+    name: str = "non_llm_string_similarity"  # type: ignore
     _required_columns: t.Dict[MetricType, t.Set[str]] = field(
         default_factory=lambda: {MetricType.SINGLE_TURN: {"reference", "response"}}
     )
     distance_measure: DistanceMeasure = DistanceMeasure.LEVENSHTEIN
 
+    def __post_init__(self):
+        try:
+            from rapidfuzz import distance
+        except ImportError:
+            raise ImportError(
+                "rapidfuzz is required for string distance. Please install it using `pip install rapidfuzz`"
+            )
+
+        self.distance_measure_map = {
+            DistanceMeasure.LEVENSHTEIN: distance.Levenshtein,
+            DistanceMeasure.HAMMING: distance.Hamming,
+            DistanceMeasure.JARO: distance.Jaro,
+        }
+
     def init(self, run_config: RunConfig):
         pass
 
@@ -81,7 +89,7 @@ async def _single_turn_ascore(
         response = sample.response
         assert isinstance(reference, str), "Expecting a string"
         assert isinstance(response, str), "Expecting a string"
-        return 1 - DISTANCE_MEASURE_MAP[self.distance_measure].normalized_distance(
+        return 1 - self.distance_measure_map[self.distance_measure].normalized_distance(
             reference, response
         )