Functions

A function is a named block of code, defined with def, that takes inputs as parameters and sends a result back with return.

Last updated: 30 Sep, 2026 · Python 3.14

The if chain in if, elif and else sorts one ticket. To sort another, you would copy it. A function gives the code a name so it can run again on different input.

def even_odd(num):
    if num%2==0:
        print("The number is even")
    else:
        print("The num is odd")

even_odd(24)

def hello_world():
    print("Hello Welcome")

val = hello_world()
print(val)

def hello_world():
    return "Hello Welcome"

val = hello_world()
print(val)

def add_number(num1, num2):
    return num1 + num2

val = add_number(24, 48)
print(val)

The number is even
Hello Welcome
None
Hello Welcome
72

The first hello_world printed its message, so val was None. The second returned it, so val held the text. The ticket version below makes the same choice: categorise returns its answer.

Syntax:

def name(parameter):
    ...
    return result

Defining categorise

def categorise(text):
    text = text.lower()
    if "charged" in text or "refund" in text:
        return "billing"
    if "parcel" in text or "arrived" in text:
        return "shipping"
    return "other"

print(categorise("I was charged twice for one order"))
print(categorise("My parcel has not arrived"))
print(categorise("How do I change my password?"))

billing
shipping
other

def starts a function. categorise is its name and text is its parameter: the name the input gets inside the function. The indented lines are what runs each time it is called.

return sends a value back to whoever called the function, and ends the function there. That is why the second if does not need an elif: once a return has run, nothing below it does.

Printing instead of returning

A function that prints its answer looks right until you try to use the answer.

def categorise_and_print(text):
    if "charged" in text:
        print("billing")

result = categorise_and_print("I was charged twice")
print("result is", result)

billing
result is None

billing appears, but result is None. A function without return gives back None, so the program cannot use what was shown. Print for people; return for code.

Calling one function from another

def label(ticket):
    category = categorise(ticket["text"])
    return f"#{ticket['id']} {category}"

print(label({"id": 3, "text": "Can I get a refund for the blue mug?"}))

#3 billing

Once categorise exists, any later code can call it. Small functions that each do one thing are easier to test, and pytest tests this exact one.

print vs return

Where functions show up in AI code

• A tool an agent can call is a plain Python function like categorise.
• Wrapping a model call once, so the rest of the program calls ask_model(text).

Related

• Previous: List comprehensions
• Next: Function arguments
• Reference: Defining Functions in the Python tutorial