LangChain / RAG / Vector DBs · 课时

将外部 API 集成为工具

将 RAG 系统连接到外部 API 和服务，使智能体能够获取实时数据或执行操作。

第 3 / 4 课11 个步骤

将外部 API 集成为工具 是 CoddyKit 上的免费 LangChain / RAG / Vector DBs 课时。这是第 3 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 LangChain / RAG / Vector DBs 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 LangChain / RAG / Vector DBs 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Agents & External APIs

Agents can do more than just generate text! They can interact with the real world by using special functions called "tools." External APIs are perfect for this, allowing agents to fetch real-time data or perform actions.

Tools for Real-World Interaction

In LangChain, a tool is essentially a Python function that an agent can call. These functions can wrap almost anything:

Searching the web
Querying a database
Sending emails
Interacting with an external API

Tools give agents superpowers, extending their capabilities beyond their internal knowledge.

Crafting an API Tool

To integrate an external API, you first define a standard Python function that makes the API call. Then, you wrap this function using LangChain's Tool class.

The Tool needs:

A name: How the agent refers to it.
A description: What the tool does and its expected input.
The actual function (func) that performs the action.

Simple Tool Function Example

Let's start with a basic Python function that simulates an API call. This function will "get current time" for a given city.

Notice how the function takes a single string argument, which the agent will provide based on its reasoning.

from langchain.tools import Tool

def get_current_time(city: str) -> str:
    """Gets the current time for a specified city."""
    # This would typically make an actual API call
    if city.lower() == "london":
        return "10:30 AM GMT"
    elif city.lower() == "new york":
        return "05:30 AM EST"
    else:
        return "Time not available for that city."

# This function will be wrapped into a Tool later.

Agent Using a Simple Tool

Here's a complete example where a LangChain agent uses our get_current_time tool. The agent decides when to call the tool based on the user's query and the tool's description.

Note: You'll need to set your OPENAI_API_KEY environment variable for this to run.

import os
from langchain.tools import Tool
from langchain_openai import OpenAI
from langchain.agents import AgentExecutor, create_react_agent
from langchain import hub

# Mock function for getting time (simulates API)
def get_current_time(city: str) -> str:
    """Gets the current time for a specified city."""
    if city.lower() == "london":
        return "10:30 AM GMT"
    elif city.lower() == "new york":
        return "05:30 AM EST"
    else:
        return "Time not available for that city."

# Create the LangChain Tool
time_tool = Tool(
    name="get_time",
    func=get_current_time,
    description="Useful for getting the current time for a given city. Input should be a city name (e.g., 'London')."
)

# List of tools available to the agent
tools = [time_tool]

# Initialize the LLM (ensure OPENAI_API_KEY is set)
llm = OpenAI(temperature=0)

# Pull the ReAct prompt template from LangChain Hub
prompt = hub.pull("hwchase17/react")

# Create the agent
agent = create_react_agent(llm, tools, prompt)

# Create the AgentExecutor to run the agent
agent_executor = AgentExecutor(agent=agent, tools=tools, verbose=True)

# Invoke the agent with a query
response = agent_executor.invoke({"input": "What time is it in London?"})
print(response["output"])

Connecting to Real APIs with `requests`

For real-world API interactions, Python's requests library is your go-to. It simplifies making HTTP requests to external services.

Within your tool function, you'll typically send a request, receive an HTTP response, and then parse its content (often JSON) to extract the relevant data.

import requests

def fetch_data_from_api(query: str) -> str:
    """Fetches data from a hypothetical external API."""
    try:
        # Example: calling a public API (e.g., a mock weather API)
        # Replace with your actual API endpoint and parameters
        api_url = f"https://api.example.com/data?q={query}" # Placeholder URL
        response = requests.get(api_url)
        response.raise_for_status() # Raise HTTPError for bad responses (4xx or 5xx)
        data = response.json()
        return str(data) # Return relevant part of the data
    except requests.exceptions.RequestException as e:
        return f"Error fetching data: {e}"

# This function would then be wrapped in a LangChain Tool.

Building a Weather Tool

Let's create a more practical example: a tool that fetches current weather for a city using a mock weather API. In a real scenario, you'd integrate with a service like OpenWeatherMap.

Notice the detailed description, guiding the LLM on the input format.

import requests
from langchain.tools import Tool

# Mock weather API function
def get_weather_for_city(city: str) -> str:
    """
    Fetches the current weather for a specified city.
    Uses a mock API for demonstration.
    """
    # In a real app, replace with an actual weather API call
    mock_weather_data = {
        "london": {"temperature": "15C", "conditions": "Cloudy"},
        "new york": {"temperature": "22C", "conditions": "Sunny"},
        "paris": {"temperature": "18C", "conditions": "Partly Cloudy"}
    }
    weather = mock_weather_data.get(city.lower())
    if weather:
        return f"The current weather in {city} is {weather['temperature']} and {weather['conditions']}."
    else:
        return f"Weather data not available for {city}."

# Create the LangChain Tool for weather
weather_tool = Tool(
    name="get_weather",
    func=get_weather_for_city,
    description="Useful for getting the current weather conditions (temperature, conditions) for a specific city. Input should be a city name (e.g., 'London')."
)

# This tool can now be added to an agent's tool list.

Agent Calling Weather API

Now, let's see an agent use our get_weather tool. The agent's reasoning process (if verbose=True) will show it deciding to use the tool, calling it, and then using the result to answer the query.

import os
from langchain.tools import Tool
from langchain_openai import OpenAI
from langchain.agents import AgentExecutor, create_react_agent
from langchain import hub

# Mock weather API function (from previous scene)
def get_weather_for_city(city: str) -> str:
    """
    Fetches the current weather for a specified city.
    Uses a mock API for demonstration.
    """
    mock_weather_data = {
        "london": {"temperature": "15C", "conditions": "Cloudy"},
        "new york": {"temperature": "22C", "conditions": "Sunny"},
        "paris": {"temperature": "18C", "conditions": "Partly Cloudy"}
    }
    weather = mock_weather_data.get(city.lower())
    if weather:
        return f"The current weather in {city} is {weather['temperature']} and {weather['conditions']}."
    else:
        return f"Weather data not available for {city}."

# Create the LangChain Tool for weather
weather_tool = Tool(
    name="get_weather",
    func=get_weather_for_city,
    description="Useful for getting the current weather conditions (temperature, conditions) for a specific city. Input should be a city name (e.g., 'London')."
)

# List of tools available to the agent
tools = [weather_tool]

# Initialize the LLM (ensure OPENAI_API_KEY is set)
llm = OpenAI(temperature=0)

# Pull the ReAct prompt template from LangChain Hub
prompt = hub.pull("hwchase17/react")

# Create the agent
agent = create_react_agent(llm, tools, prompt)

# Create the AgentExecutor to run the agent
agent_executor = AgentExecutor(agent=agent, tools=tools, verbose=True)

# Invoke the agent with a query
response = agent_executor.invoke({"input": "What's the weather like in Paris?"})
print(response["output"])

Effective Tool Descriptions

The LLM relies heavily on your tool's description to decide if and how to use it. Make your descriptions:

Clear and concise: State exactly what the tool does.
Specific about input: Clearly explain what kind of argument it expects.
Purpose-driven: Explain *why* the tool is useful or in what scenarios.
Provide examples: If the input is complex, offer an example format.

A well-crafted description is crucial for an agent's success.

Tool Integration Check

You're building an agent to help users find real-time information. You need to create a tool that fetches current stock prices for a given company ticker (e.g., "AAPL") by calling an external API.

Which of the following describes the most important aspect of the Tool definition for the LLM to understand and use it correctly?

在交互式课程中回答

Recap: APIs as Agent Tools

You've learned how to empower LangChain agents by integrating external APIs as tools. This allows your agents to:

Access real-time data from the internet.
Perform actions in the real world (like sending emails or updating databases).
Extend their capabilities far beyond their initial training data.

By wrapping API calls in well-described Tool objects, you significantly enhance your agent's intelligence and utility!

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将 RAG 系统连接到外部 API 和服务，使智能体能够获取实时数据或执行操作。你通过在浏览器中直接运行的动手代码来练习 LangChain / RAG / Vector DBs，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

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