Phase 1 · Day 3 · Learn AI

Defining and calling functions

A function is a named block of code. You define it with def, then call it by name with parentheses.

Use snake_case for function names: calculate_total, greet_user. Keep each function focused on one job.

A simple function

def say_hello():
    print("Hello, Learn AI!")

say_hello()
say_hello()

Remember the colon after the header and indent the body—same rules as if and for.

Parameters & arguments

Parameters are names listed in the def line. Arguments are the values you pass when calling.

You can pass multiple arguments in order (positional) or by name (keyword).

Parameters in action

def greet(name):
    print(f"Welcome, {name}!")

greet("Ada")
greet("Grace")

def introduce(name, role):
    print(f"{name} is a {role}.")

introduce("Ada", "engineer")
introduce(role="researcher", name="Grace")

Keyword arguments make calls clearer when there are several parameters.

Returning values

print shows something; return sends a value back to the caller so you can store or reuse it.

A function without return (or with a bare return) gives back None.

return vs print

def add(a, b):
    return a + b

total = add(3, 5)
print(total)          # 8
print(add(10, 2) * 2) # 24

def shout(text):
    print(text.upper())

result = shout("hi")
print(result)  # None — shout only prints

Prefer return when the next step needs the result. Prefer print for display-only helpers.

Defaults & local scope

Default parameter values make arguments optional. Put parameters with defaults after required ones.

Variables created inside a function are local—they do not leak into the rest of your program.

Default arguments

def power(base, exponent=2):
    return base ** exponent

print(power(5))      # 25 (square)
print(power(2, 8))   # 256

Local scope

def double(n):
    result = n * 2
    return result

print(double(4))
# print(result)  # NameError — result exists only inside double

Avoid mutable defaults like [] or {}—use None and create a new list/dict inside the function instead.

Practice

Mini challenge: tip calculator

Write a function calculate_tip(bill, percent=15) that returns the tip amount. Then write total_with_tip(bill, percent=15) that returns bill + tip. Ask the user for a bill and print both values.

challenge starter

def calculate_tip(bill, percent=15):
    # TODO: return the tip amount
    pass

def total_with_tip(bill, percent=15):
    # TODO: return bill + tip (reuse calculate_tip)
    pass

bill = float(input("Bill amount: "))
# print tip and total

Interactive

Try It in the browser

Write is_even(n) that returns True if n is even, otherwise False. Ask for a number and print the result.

Output

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Errors

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Functions