Let AI Call Your API (Function Calling Tutorial)

#ai #llm #api #python

Function calling lets AI decide when to call your API. Here's how to implement it in 30 lines of code — with real examples.

AI is great at generating text. But you know what's better? AI that can take actions.

Function calling lets AI decide: "I need to call this API to answer the user's question."

Here's how to implement it.

The Setup

Let's say you have a weather API. Instead of AI making up the weather, it can call your API to get real data.

Step 1: Define Your Functions

functions = [
    {
        "name": "get_weather",
        "description": "Get current weather for a city",
        "parameters": {
            "type": "object",
            "properties": {
                "city": {"type": "string", "description": "City name"}
            },
            "required": ["city"]
        }
    }
]

Step 2: Let AI Decide

response = client.chat.completions.create(
    model="deepseek-v4-pro",
    messages=[{"role": "user", "content": "What's the weather in Tokyo?"}],
    tools=[{"type": "function", "function": f} for f in functions]
)

# AI decides to call get_weather("Tokyo")
message = response.choices[0].message

Step 3: Execute the Function

if message.tool_calls:
    # AI wants to call a function
    func_name = message.tool_calls[0].function.name
    args = json.loads(message.tool_calls[0].function.arguments)

    if func_name == "get_weather":
        result = requests.get(f"https://api.weather.com/v1/{args['city']}").json()

        # Send result back to AI
        final_response = client.chat.completions.create(
            model="deepseek-v4-pro",
            messages=[
                {"role": "user", "content": "What's the weather in Tokyo?"},
                message,
                {"role": "tool", "tool_call_id": message.tool_calls[0].id, 
                 "content": json.dumps(result)}
            ]
        )
        print(final_response.choices[0].message.content)

Real Example: AI Assistant with Tools

# Full example: AI that can check weather, search DB, and send emails
functions = [get_weather, query_database, send_email]

while True:
    user_input = input("You: ")

    response = client.chat.completions.create(
        model="deepseek-v4-pro",
        messages=[{"role": "user", "content": user_input}],
        tools=[{"type": "function", "function": f} for f in functions]
    )

    # AI automatically calls the right function
    # You just execute it and return the result