A function that always does exactly the same thing is limited. Parameters let you pass data in.

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

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

name is a parameter — a variable that exists inside the function. "Ada" is an argument — the actual value you pass when calling. The parameter is the slot; the argument is what you put in it.

Multiple parameters

Separate them with commas. Arguments are matched to parameters by position.

def describe(name, age):
    print(f"{name} is {age} years old.")

describe("Ada", 36)

Get the order wrong and you get nonsense — describe(36, "Ada") would print "36 is Ada years old." You can be explicit with keyword arguments, which work in any order:

def describe(name, age):
    print(f"{name} is {age} years old.")

describe(age=36, name="Ada")

Returning a value

Printing shows something to the user. Returning hands a value back to the code that called the function, so it can be stored, reused, or passed along.

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

result = add(3, 5)
print(result)
print(add(10, 20))

This distinction matters more than it first appears:

def add_and_print(a, b):
    print(a + b)

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

x = add_and_print(3, 5)
y = add_and_return(3, 5)

print(x)
print(y)

add_and_print displays 8 but gives back nothing, so x is None. add_and_return gives back 8, so y is 8 and you can keep working with it. As a rule: functions should return values; let the caller decide whether to print.

return exits immediately

Nothing after a return runs:

def check(number):
    if number > 0:
        return "positive"
    return "zero or negative"

print(check(5))
print(check(-3))

Because return exits, you often don't need an else.

Returning multiple values

Return several values separated by commas and Python packs them into a tuple — which you can unpack, exactly as in lesson 3.4:

def min_max(numbers):
    return min(numbers), max(numbers)

low, high = min_max([3, 1, 4, 1, 5])
print(low)
print(high)

Try it yourself

Write add(a, b) that returns the sum, and multiply(a, b) that returns the product. Print both results.

Checkpoint

• Parameters are the named slots; arguments are the values passed in.
• Positional arguments match by order; keyword arguments match by name.
• return sends a value back and exits the function immediately.
• A function with no return gives back None.
• Prefer returning over printing — the caller can always print.