Programming Fundamentals › Programming Basics

Default Parameter

A parameter value used when the caller omits it.

Also known as: default argument, default parameters, default value, optional parameter

A default parameter gives an argument a fallback value that’s used when the caller doesn’t supply one.

def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

greet("Ana")              # "Hello, Ana!"
greet("Ana", "Hi")        # "Hi, Ana!"

function greet(name, greeting = "Hello") { ... }

Defaults make a function easy to call in the common case, and still customizable.

Rules

• Parameters with defaults usually go last (in Python it’s required: a non-default can’t follow a default).
• Use keyword arguments to skip over some: connect(host, timeout=5).

The famous Python trap: mutable defaults

The default value is created once, when the function is defined, not on each call:

def add_item(item, items=[]):     # BUG: one shared list
    items.append(item)
    return items

add_item(1)   # [1]
add_item(2)   # [1, 2]   surprise!

def add_item(item, items=None):   # fix
    if items is None:
        items = []
    items.append(item)
    return items

Never use a list, dictionary or set as a default. Use None and create it inside (mutable vs immutable).

In JavaScript

Defaults apply when the argument is undefined (missing), not for null (undefined):

function f(x = 5) { return x; }
f();           // 5
f(undefined);  // 5
f(null);       // null

Habits

• Pick safe, obvious defaults, such as 30 seconds or False.
• Don’t use many defaults in one signature. Group them into an options object.
• Beware of boolean defaults that flip behavior (flag arguments).