How does function calling (tool use) work in the Gemini API?

from google import genai
from google.genai import types
import json

client = genai.Client()

# 1. Declare your functions as tools
get_weather = types.FunctionDeclaration(
    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 may request a function call
response = client.models.generate_content(
    model="gemini-3.5-flash",
    contents="What's the weather in Tokyo right now?",
    config=types.GenerateContentConfig(
        tools=[types.Tool(function_declarations=[get_weather])]
    )
)

# 3. Check if model requested a function call
for part in response.candidates[0].content.parts:
    if hasattr(part, "function_call"):
        fc = part.function_call
        print(f"Model wants to call: {fc.name}({dict(fc.args)})")
        result = your_weather_api(fc.args["city"])   # execute!

        # 4. Return result and continue
        response2 = client.models.generate_content(
            model="gemini-3.5-flash",
            contents=[
                types.Content(role="user", parts=[types.Part(text="What's the weather in Tokyo?")]),
                response.candidates[0].content,  # model turn with function_call
                types.Content(
                    role="user",
                    parts=[types.Part(
                        function_response=types.FunctionResponse(
                            name=fc.name,
                            response={"result": result}
                        )
                    )]
                )
            ],
        )
        print(response2.text)