Default Parameter Values

Default parameters allow functions to be called with fewer arguments, making them more flexible and easier to use with sensible defaults.

Default parameters provide fallback values when arguments aren't supplied. This makes functions more flexible and user-friendly.

Basic Default Parameter:

def greet(name="World"):

    return f"Hello, {name}!"


greet()         # "Hello, World!" (uses default)

greet("Alice")  # "Hello, Alice!" (overrides default)

Multiple Defaults:

def create_user(username, role="user", active=True):

    return {

        "username": username,

        "role": role,

        "active": active

    }


create_user("alice")  # All defaults

create_user("bob", "admin")  # Override role

create_user("charlie", active=False)  # Named override

Rules for Default Parameters:

1. Defaults Must Come After Required:

# ❌ WRONG - SyntaxError

def func(a=1, b):

    pass


# ✅ CORRECT

def func(b, a=1):

    pass

2. Evaluated Once at Definition:

def func(x=5):  # 5 is evaluated when function is defined

    return x

3. Mutable Default Values - DANGER!

# ❌ DANGEROUS - Mutable default

def add_item(item, list=[]):

    list.append(item)

    return list


add_item(1)  # [1]

add_item(2)  # [1, 2] - UNEXPECTED! Same list object

# ✅ CORRECT - Use None

def add_item(item, list=None):

    if list is None:

        list = []

    list.append(item)

    return list

Common Patterns:

Configuration Functions:

def connect_db(host="localhost", port=5432, timeout=30):

    # Connection logic

    pass

API Calls:

def fetch_data(url, method="GET", timeout=10, headers=None):

    if headers is None:

        headers = {}

    # Fetch logic

String Processing:

def process_text(text, lowercase=True, strip=True):

    if lowercase:

        text = text.lower()

    if strip:

        text = text.strip()

    return text

Default from Expression:

import datetime


def log_message(msg, timestamp=None):

    if timestamp is None:

        timestamp = datetime.datetime.now()

    print(f"[{timestamp}] {msg}")

Best Practices:

• Use immutable defaults (numbers, strings, None, tuples)


• Never use mutable defaults (lists, dicts, sets)


• Defaults should be sensible, commonly-used values


• Document default behavior in docstrings