Python - Functions Basics

Overview

Estimated time: 35–45 minutes

Define functions, choose parameter styles, avoid mutable default pitfalls, and document with docstrings.

Learning Objectives

• Define and call functions with positional and keyword arguments.
• Use parameter kinds (positional-only, keyword-only, *args, **kwargs) appropriately.
• Avoid mutable default arguments; write clear docstrings.

Prerequisites

• Python - Control Flow

Defining functions

def add(a, b):
    """Return the sum of a and b."""
    return a + b

print(add(2, 3))
print(add(a=2, b=3))

Expected Output: 5\n5

Defaults and pitfalls

def bad_append(x, items=[]):  # don't do this
    items.append(x)
    return items

print(bad_append(1))
print(bad_append(2))  # unexpected: shares list

# Correct approach
def good_append(x, items=None):
    if items is None:
        items = []
    items.append(x)
    return items

Expected Output (first two lines): [1]\n[1, 2]

Common Pitfalls

• Mutable default arguments that persist across calls.
• Overusing *args/**kwargs leading to unclear APIs.

Checks for Understanding

1. How do you safely provide a list default parameter?
2. What are two benefits of keyword arguments?

Exercises

1. Write a function with a keyword-only parameter and call it by name.
2. Refactor a function to avoid using *args/**kwargs when not necessary.