Foundation project

Project: an inventory manager

Lesson 12 of 20 · 20 min · Module 3: Objects that behave

Combine classes, dictionaries, validation, and properties to build the core of a small shop’s inventory system.

Free. No installation: you write and run the code in your browser, and it is checked automatically.

By the end, you’ll be able to…

Design a class around one internal data structure
Validate before changing state
Provide read-only views with properties

The idea

Keep the stock in a dictionary inside the object. Every method either changes the stock safely or reports on it. Invalid operations raise ValueError with a clear message and leave the stock unchanged.

stock = {"apples": 5}
stock["apples"] = stock.get("apples", 0) + 3
print(stock)

01Hide the data, expose behaviour

The dictionary lives in self._stock. Callers use add, remove, and report, so the rules (no negative stock, no zero entries) are enforced in one place.

02Validate first, change second

If remove checked the quantity after subtracting, a failed call could leave the stock broken. Checking every condition before touching the dict keeps the object consistent even when errors happen.

inv = Inventory()
inv.add("ink", 2)
try:
    inv.remove("ink", 5)
except ValueError as err:
    print(err, inv.report())

Output

not enough ink ['ink: 2']

03bool is an int

In Python, True == 1 and isinstance(True, int) is True. Explicitly rejecting bool stops add("x", True) from adding one unit by accident.

Your project brief

Write a class Inventory with:
• add(name, quantity): adds stock; quantity must be a positive integer, otherwise raise ValueError;
• remove(name, quantity): removes stock; raise ValueError if the item is unknown or there is not enough; remove the item entirely when it reaches 0;
• a total property with the total number of units;
• low_stock(threshold): sorted names with quantity below threshold;
• report(): lines "apples: 5" sorted by name.

What the checks look for

Adds stock and reports it sorted
Removes stock and deletes items that reach zero
Rejects invalid quantities
Rejects removing unknown or insufficient stock without changing anything
Calculates the total and low stock

Try it in the editor →

Take this with you

A good class protects its own rules: check, then change.

Go a little further

Add a history list that records every successful add and remove, and a method that undoes the last change.