Support Streaming Tokens from OpenAI (#364)

https://github.com/hwchase17/langchain/issues/363

@hwchase17 how much does this make you want to cry?

langchain/llms/openai.py

@@ -1,6 +1,6 @@
 """Wrapper around OpenAI APIs."""
 import sys
-from typing import Any, Dict, List, Mapping, Optional
+from typing import Any, Dict, Generator, List, Mapping, Optional
 
 from pydantic import BaseModel, Extra, Field, root_validator
 
@@ -160,6 +160,30 @@ class OpenAI(LLM, BaseModel):
             generations=generations, llm_output={"token_usage": token_usage}
         )
 
+    def stream(self, prompt: str) -> Generator:
+        """Call OpenAI with streaming flag and return the resulting generator.
+
+        Args:
+            prompt: The prompts to pass into the model.
+
+        Returns:
+            A generator representing the stream of tokens from OpenAI.
+
+        Example:
+            .. code-block:: python
+
+                generator = openai.stream("Tell me a joke.")
+                for token in generator:
+                    yield token
+        """
+        params = self._default_params
+        if params["best_of"] != 1:
+            raise ValueError("OpenAI only supports best_of == 1 for streaming")
+        params["stream"] = True
+        generator = self.client.create(model=self.model_name, prompt=prompt, **params)
+
+        return generator
+
     @property
     def _identifying_params(self) -> Mapping[str, Any]:
         """Get the identifying parameters."""

tests/integration_tests/llms/test_openai.py

@@ -1,6 +1,7 @@
 """Test OpenAI API wrapper."""
 
 from pathlib import Path
+from typing import Generator
 
 import pytest
 
@@ -55,3 +56,21 @@ def test_saving_loading_llm(tmp_path: Path) -> None:
     llm.save(file_path=tmp_path / "openai.yaml")
     loaded_llm = load_llm(tmp_path / "openai.yaml")
     assert loaded_llm == llm
+
+
+def test_openai_streaming() -> None:
+    """Test streaming tokens from OpenAI."""
+    llm = OpenAI(max_tokens=10)
+    generator = llm.stream("I'm Pickle Rick")
+
+    assert isinstance(generator, Generator)
+
+    for token in generator:
+        assert isinstance(token["choices"][0]["text"], str)
+
+
+def test_openai_streaming_error() -> None:
+    """Test error handling in stream."""
+    llm = OpenAI(best_of=2)
+    with pytest.raises(ValueError):
+        llm.stream("I'm Pickle Rick")