Gracefully degrade when model asks for nonexistent tool (#268)

Not yet tested, but very simple change, assumption is that we're cool
with just producing a generic output when tool is not found

tests/unit_tests/agents/test_agent.py

@@ -0,0 +1,45 @@
+"""Unit tests for agents."""
+
+from typing import Any, List, Mapping, Optional
+
+from langchain.agents import Tool, initialize_agent
+from langchain.llms.base import LLM
+
+
+class FakeListLLM(LLM):
+    """Fake LLM for testing that outputs elements of a list."""
+
+    def __init__(self, responses: List[str]):
+        """Initialize with list of responses."""
+        self.responses = responses
+        self.i = -1
+
+    def __call__(self, prompt: str, stop: Optional[List[str]] = None) -> str:
+        """Increment counter, and then return response in that index."""
+        self.i += 1
+        print(self.i)
+        print(self.responses)
+        return self.responses[self.i]
+
+    @property
+    def _identifying_params(self) -> Mapping[str, Any]:
+        return {}
+
+
+def test_agent_bad_action() -> None:
+    """Test react chain when bad action given."""
+    bad_action_name = "BadAction"
+    responses = [
+        f"I'm turning evil\nAction: {bad_action_name}\nAction Input: misalignment",
+        "Oh well\nAction: Final Answer\nAction Input: curses foiled again",
+    ]
+    fake_llm = FakeListLLM(responses)
+    tools = [
+        Tool("Search", lambda x: x, "Useful for searching"),
+        Tool("Lookup", lambda x: x, "Useful for looking up things in a table"),
+    ]
+    agent = initialize_agent(
+        tools, fake_llm, agent="zero-shot-react-description", verbose=True
+    )
+    output = agent.run("when was langchain made")
+    assert output == "curses foiled again"

tests/unit_tests/agents/test_react.py

@@ -2,8 +2,6 @@
 
 from typing import Any, List, Mapping, Optional, Union
 
-import pytest
-
 from langchain.agents.react.base import ReActChain, ReActDocstoreAgent
 from langchain.agents.tools import Tool
 from langchain.docstore.base import Docstore
@@ -94,10 +92,12 @@ def test_react_chain() -> None:
 
 def test_react_chain_bad_action() -> None:
     """Test react chain when bad action given."""
+    bad_action_name = "BadAction"
     responses = [
-        "I should probably search\nAction 1: BadAction[langchain]",
+        f"I'm turning evil\nAction 1: {bad_action_name}[langchain]",
+        "Oh well\nAction 2: Finish[curses foiled again]",
     ]
     fake_llm = FakeListLLM(responses)
     react_chain = ReActChain(llm=fake_llm, docstore=FakeDocstore())
-    with pytest.raises(KeyError):
-        react_chain.run("when was langchain made")
+    output = react_chain.run("when was langchain made")
+    assert output == "curses foiled again"