add stop to stream (#729)

Author: hwchase17

langchain/llms/openai.py

@@ -182,14 +182,15 @@ def _generate(
             generations=generations, llm_output={"token_usage": token_usage}
         )
 
-    def stream(self, prompt: str) -> Generator:
+    def stream(self, prompt: str, stop: Optional[List[str]] = None) -> Generator:
         """Call OpenAI with streaming flag and return the resulting generator.
 
         BETA: this is a beta feature while we figure out the right abstraction.
         Once that happens, this interface could change.
 
         Args:
             prompt: The prompts to pass into the model.
+            stop: Optional list of stop words to use when generating.
 
         Returns:
             A generator representing the stream of tokens from OpenAI.
@@ -204,6 +205,10 @@ def stream(self, prompt: str) -> Generator:
         params = self._invocation_params
         if params["best_of"] != 1:
             raise ValueError("OpenAI only supports best_of == 1 for streaming")
+        if stop is not None:
+            if "stop" in params:
+                raise ValueError("`stop` found in both the input and default params.")
+            params["stop"] = stop
         params["stream"] = True
         generator = self.client.create(prompt=prompt, **params)