community[patch]: give reranker default client val (#17289)

libs/langchain/langchain/retrievers/document_compressors/cohere_rerank.py

@@ -14,14 +14,15 @@ from langchain.utils import get_from_dict_or_env
 class CohereRerank(BaseDocumentCompressor):
     """Document compressor that uses `Cohere Rerank API`."""
 
-    client: Any
+    client: Any = None
     """Cohere client to use for compressing documents."""
     top_n: Optional[int] = 3
     """Number of documents to return."""
     model: str = "rerank-english-v2.0"
     """Model to use for reranking."""
-
     cohere_api_key: Optional[str] = None
+    """Cohere API key. Must be specified directly or via environment variable 
+        COHERE_API_KEY."""
     user_agent: str = "langchain"
     """Identifier for the application making the request."""
 
@@ -34,18 +35,19 @@ class CohereRerank(BaseDocumentCompressor):
     @root_validator(pre=True)
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that api key and python package exists in environment."""
-        try:
-            import cohere
-        except ImportError:
-            raise ImportError(
-                "Could not import cohere python package. "
-                "Please install it with `pip install cohere`."
+        if not values.get("client"):
+            try:
+                import cohere
+            except ImportError:
+                raise ImportError(
+                    "Could not import cohere python package. "
+                    "Please install it with `pip install cohere`."
+                )
+            cohere_api_key = get_from_dict_or_env(
+                values, "cohere_api_key", "COHERE_API_KEY"
             )
-        cohere_api_key = get_from_dict_or_env(
-            values, "cohere_api_key", "COHERE_API_KEY"
-        )
-        client_name = values.get("user_agent", "langchain")
-        values["client"] = cohere.Client(cohere_api_key, client_name=client_name)
+            client_name = values.get("user_agent", "langchain")
+            values["client"] = cohere.Client(cohere_api_key, client_name=client_name)
         return values
 
     def rerank(