refactor: Refactored how to disable stemming in bm25

fastembed/sparse/bm25.py

@@ -92,6 +92,7 @@ class Bm25(SparseTextEmbeddingBase):
         b (float, optional): The b parameter in the BM25 formula. Defines the importance of the document length.
             Defaults to 0.75.
         avg_len (float, optional): The average length of the documents in the corpus. Defaults to 256.0.
+        language (str, optional): Specifies the language for the stemmer. Set to None to disable stemming.
     Raises:
         ValueError: If the model_name is not in the format <org>/<model> e.g. BAAI/bge-base-en.
     """
@@ -103,14 +104,13 @@ def __init__(
         k: float = 1.2,
         b: float = 0.75,
         avg_len: float = 256.0,
-        language: str = "english",
+        language: Optional[str] = "english",
         token_max_length: int = 40,
-        disable_stemmer: bool = False,
         **kwargs,
     ):
         super().__init__(model_name, cache_dir, **kwargs)
 
-        if language not in supported_languages:
+        if language is not None and language not in supported_languages:
             raise ValueError(f"{language} language is not supported")
         else:
             self.language = language
@@ -130,8 +130,7 @@ def __init__(
         self.punctuation = set(get_all_punctuation())
         self.stopwords = set(self._load_stopwords(self._model_dir, self.language))
 
-        self.disable_stemmer = disable_stemmer
-        self.stemmer = SnowballStemmer(language) if not disable_stemmer else None
+        self.stemmer = SnowballStemmer(language)
         self.tokenizer = SimpleTokenizer
 
     @classmethod
@@ -144,7 +143,7 @@ def list_supported_models(cls) -> list[dict[str, Any]]:
         return supported_bm25_models
 
     @classmethod
-    def _load_stopwords(cls, model_dir: Path, language: str) -> list[str]:
+    def _load_stopwords(cls, model_dir: Path, language: Optional[str]) -> list[str]:
         stopwords_path = model_dir / f"{language}.txt"
         if not stopwords_path.exists():
             return []
@@ -225,9 +224,6 @@ def embed(
         )
 
     def _stem(self, tokens: list[str]) -> list[str]:
-        if self.disable_stemmer:
-            return tokens
-
         stemmed_tokens = []
         for token in tokens:
             if token in self.punctuation:
@@ -239,7 +235,10 @@ def _stem(self, tokens: list[str]) -> list[str]:
             if len(token) > self.token_max_length:
                 continue
 
-            stemmed_token = self.stemmer.stem_word(token.lower())
+            if self.stemmer:
+                stemmed_token = self.stemmer.stem_word(token.lower())
+            else:
+                stemmed_token = token.lower()
 
             if stemmed_token:
                 stemmed_tokens.append(stemmed_token)