Parameters

Functions become much more useful when they can accept inputs. Inputs allow the same function to work with different values each time it is called.

The Problem

Consider this function from the previous page:

def greet():
    print("=" * 30)
    print("  Welcome, Alice")
    print("=" * 30)

greet()
greet()

Output

==============================
  Welcome, Alice
==============================
==============================
  Welcome, Alice
==============================

This function always welcomes Alice. What if we want to welcome different people?

The Solution

We can add a parameter so the function accepts a value.

def greet(name):
    print("=" * 30)
    print("  Welcome,", name)
    print("=" * 30)

greet("Alice")
greet("Bob")

Output

==============================
  Welcome, Alice
==============================
==============================
  Welcome, Bob
==============================

The function now accepts a value that can change each time it is called. When we call greet("Alice"), Python assigns "Alice" to the parameter name inside the function body.

greet("Alice")
       │
       └──► name = "Alice"

Parameters and Arguments

A parameter is a variable listed in the function definition.

An argument is the value supplied when the function is called.

def greet(name):
    print("Welcome,", name)

greet("Alice")

Here:

• name is the parameter
• "Alice" is the argument

Multiple Parameters

Functions can accept more than one parameter.

def describe(name, age):
    print(name, "is", age, "years old")

describe("Alice", 25)
describe("Bob", 30)

Output

Alice is 25 years old
Bob is 30 years old

Each parameter receives its own value when the function is called. The order of the arguments must match the order of the parameters.

Default Parameters

Parameters can have default values.

def greet(name="guest"):
    print("Welcome,", name)

greet()
greet("Alice")

Output

Welcome, guest
Welcome, Alice

When no argument is provided, the parameter uses its default value. When an argument is provided, it overrides the default.

Key Ideas

Parameters let a function accept different inputs each time it is called. The parameter name appears in the def line; the argument value appears in the call. When a function has multiple parameters, arguments are matched to parameters by position.

Next: Return Values.