What is function calling (tool use) in the OpenAI API and how does it work?

# Chat Completions - function calling
from openai import OpenAI
import json

client = OpenAI()

# 1. Define functions as tools
tools = [{
    "type": "function",
    "function": {
        "name": "get_weather",
        "description": "Get current weather for a city",
        "parameters": {
            "type": "object",
            "properties": {
                "city": {"type": "string", "description": "City name"},
                "unit": {"type": "string", "enum": ["celsius", "fahrenheit"]}
            },
            "required": ["city"]
        }
    }
}]

# 2. First call - model decides to call a function
response = client.chat.completions.create(
    model="gpt-5.5",
    messages=[{"role": "user", "content": "What's the weather in Tokyo?"}],
    tools=tools,
    tool_choice="auto",  # "auto" | "required" | {"type":"function","function":{"name":"..."}}
)

# 3. Extract the function call
message = response.choices[0].message
tool_call = message.tool_calls[0]
function_args = json.loads(tool_call.function.arguments)

# 4. Execute your function
weather_result = get_weather(**function_args)  # your actual implementation

# 5. Second call - send function result back
response2 = client.chat.completions.create(
    model="gpt-5.5",
    messages=[
        {"role": "user", "content": "What's the weather in Tokyo?"},
        message,  # assistant turn with tool_calls
        {"role": "tool", "content": json.dumps(weather_result), "tool_call_id": tool_call.id}
    ],
    tools=tools,
)