Phase 1: Programming Fundamentals I · ~40 minutes · Python · uv

Functions — Abstraction and Reuse

A function that print()s its answer looks like it works right up until something tries to use that answer — and gets None.

Hiring signal: Immediately recognizes a None-related TypeError as a missing return, not a mysterious failure

What you will learn

Explain why wrapping code in a function is different from just running the same code inline
Distinguish return from print, and explain why a function that only prints silently breaks anything that uses its result
Use parameters, arguments, and default argument values correctly
Explain local vs. global scope well enough to predict what a function can and can't affect outside itself

Introduction

Type: Learn Languages: Python Prerequisites: Lesson 03 (Control Flow — Loops) Time: ~40 minutes

Objective

Learning objectives

Explain why wrapping code in a function is different from just running the same code inline
Distinguish return from print, and explain why a function that only prints silently breaks anything that uses its result
Use parameters, arguments, and default argument values correctly
Explain local vs. global scope well enough to predict what a function can and can't affect outside itself

What you're building

Take this (deliberately repeated) block, copy-pasted three times with only the numbers changed:

quantity = 3
price = quantity * 4.50
tax = price * 0.08
total = price + tax
print(f"Total: ${total:.2f}")

quantity = 7
price = quantity * 4.50
tax = price * 0.08
total = price + tax
print(f"Total: ${total:.2f}")

quantity = 12
price = quantity * 4.50
tax = price * 0.08
total = price + tax
print(f"Total: ${total:.2f}")

A script (refactor_totals.py) that:

Refactors this into one function, calculate_total(quantity, price_per_item=4.50, tax_rate=0.08), using return (not print) for the computed total
Calls it three times with the original quantities (3, 7, 12) and prints each result at the call site, not inside the function
Contains a comment showing the broken print-only version of the function and a one-sentence diagnosis of exactly what error calling it inside an expression would produce

A function is defined as:

def make_username(first_name, last_name, separator="."):
    return f"{first_name}{separator}{last_name}".lower()

What does make_username("Ada", "Lovelace") return?

separator has a default value ("."), so omitting it is completely valid — it isn't an error, and this is precisely what default arguments are for. The function does have a return statement, so the "no explicit print" answer describes a different bug from Lesson 01-04's actual example, not this one — return (not print) is exactly the correct way to hand a value back, so this function is not broken. Tracing the actual computation: f"{first_name}{separator}{last_name}" produces "Ada.Lovelace", and .lower() converts every letter to lowercase, giving "ada.lovelace".

Unlock the full lesson

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