What is function calling? Meaning and example in Python

What is function calling?

Function calling (also called Tool calling, Tool use): A feature where the model, instead of plain text, returns a request to run a named function with arguments in JSON that your code then executes. You describe available functions in advance. It connects language models to databases, APIs and calculators.

Function calling: an example

Given get_weather(city), the model answers a Bengaluru weather question with {"name":"get_weather","arguments":{"city":"Bengaluru"}}.

Function calling in Python

A short example you can run (Python 3.11). The output below is what it really prints.

import json

tool_schema = {"name": "convert_currency", "parameters": {"type": "object",
               "properties": {"amount": {"type": "number"}, "to": {"type": "string"}}, "required": ["amount", "to"]}}

def convert_currency(amount, to):
    rates = {"USD": 0.012, "EUR": 0.011}      # illustrative rates
    return round(amount * rates[to], 2)

# What a model returns when it decides to call the tool:
model_reply = '{"name": "convert_currency", "arguments": {"amount": 29999, "to": "USD"}}'
call = json.loads(model_reply)
print(tool_schema["name"], "->", convert_currency(**call["arguments"]))

Output

convert_currency -> 359.99

Related terms

More Prompt context engineering terms

Where you learn it

Function calling is part of the AI Engineering Program, which teaches it with notes, a hands-on lab and an exam. The first lesson is free.