Pass kwargs from initialize_agent into agent classmethod (#799)

# Problem
I noticed that in order to change the prefix of the prompt in the
`zero-shot-react-description` agent
we had to dig around to subset strings deep into the agent's attributes.
It requires the user to inspect a long chain of attributes and classes.

`initialize_agent -> AgentExecutor -> Agent -> LLMChain -> Prompt from
Agent.create_prompt`

``` python
agent = initialize_agent(
    tools=tools,
    llm=fake_llm,
    agent="zero-shot-react-description"
)
prompt_str = agent.agent.llm_chain.prompt.template
new_prompt_str = change_prefix(prompt_str)
agent.agent.llm_chain.prompt.template = new_prompt_str
```

# Implemented Solution

`initialize_agent` accepts `**kwargs` but passes it to `AgentExecutor`
but not `ZeroShotAgent`, by simply giving the kwargs to the agent class
methods we can support changing the prefix and suffix for one agent
while allowing future agents to take advantage of `initialize_agent`.


```
agent = initialize_agent(
    tools=tools,
    llm=fake_llm,
    agent="zero-shot-react-description",
    agent_kwargs={"prefix": prefix, "suffix": suffix}
)
```

To be fair, this was before finding docs around custom agents here:
https://langchain.readthedocs.io/en/latest/modules/agents/examples/custom_agent.html?highlight=custom%20#custom-llmchain
but i find that my use case just needed to change the prefix a little.


# Changes

* Pass kwargs to Agent class method
* Added a test to check suffix and prefix

---------

Co-authored-by: Jason Liu <jason@jxnl.coA>

langchain/agents/initialize.py

@@ -14,6 +14,7 @@ def initialize_agent(
     agent: Optional[str] = None,
     callback_manager: Optional[BaseCallbackManager] = None,
     agent_path: Optional[str] = None,
+    agent_kwargs: Optional[dict] = None,
     **kwargs: Any,
 ) -> AgentExecutor:
     """Load agent given tools and LLM.
@@ -50,8 +51,9 @@ def initialize_agent(
                 f"Valid types are: {AGENT_TO_CLASS.keys()}."
             )
         agent_cls = AGENT_TO_CLASS[agent]
+        agent_kwargs = agent_kwargs or {}
         agent_obj = agent_cls.from_llm_and_tools(
-            llm, tools, callback_manager=callback_manager
+            llm, tools, callback_manager=callback_manager, **agent_kwargs
         )
     elif agent_path is not None:
         agent_obj = load_agent(

tests/unit_tests/agents/test_agent.py

@@ -196,3 +196,29 @@ def test_agent_tool_return_direct() -> None:
 
     output = agent.run("when was langchain made")
     assert output == "misalignment"
+
+
+def test_agent_with_new_prefix_suffix() -> None:
+    """Test agent initilization kwargs with new prefix and suffix."""
+    fake_llm = FakeListLLM(
+        responses=["FooBarBaz\nAction: Search\nAction Input: misalignment"]
+    )
+    tools = [
+        Tool("Search", lambda x: x, "Useful for searching", return_direct=True),
+    ]
+    prefix = "FooBarBaz"
+
+    suffix = "Begin now!\nInput: {input}\nThought: {agent_scratchpad}"
+
+    agent = initialize_agent(
+        tools=tools,
+        llm=fake_llm,
+        agent="zero-shot-react-description",
+        agent_kwargs={"prefix": prefix, "suffix": suffix},
+    )
+
+    # avoids "BasePromptTemplate" has no attribute "template" error
+    assert hasattr(agent.agent.llm_chain.prompt, "template")
+    prompt_str = agent.agent.llm_chain.prompt.template
+    assert prompt_str.startswith(prefix), "Prompt does not start with prefix"
+    assert prompt_str.endswith(suffix), "Prompt does not end with suffix"