Basic usage
Input
InventoryItem.set_tax_rate(0.2); InventoryItem(100).price_with_tax()
Output
120.0
Setting the class tax_rate to 0.2 makes a 100 unit item cost 120.0 with tax.
Full lesson preview
Update Shared Class State with a Class Method
Use a class method to update class-level state that affects instances of that class. Learn how classmethods are bound to the calling class, making subclass behavior predictable.
Python practice25 minMethods & PropertiesAdvancedLast updated April 7, 2026
Problem statement
You will implement InventoryItem with a class-level tax_rate and a class method set_tax_rate that updates the tax_rate for the class (cls) the method is called on. Instances should compute price_with_tax() using the tax_rate of their actual class. This demonstrates how class methods operate on the calling class rather than always on the base class. The class method should return the new tax_rate after updating.
Task
Implement a class method that updates a class variable (shared state) and ensure instance behavior reflects the class that was updated (base class vs subclass).
Examples
Input format
Each test is a single Python expression evaluated after your class code runs. Use class methods and instance methods in expressions.
Output format
Return values (numbers or tuples) from expressions; the harness compares their string representation.
Constraints
Do not use global variables to store per-class tax rates. The classmethod must set the tax_rate on cls so subclasses can maintain independent tax rates. Tax rates are floats; price is treated as float. price_with_tax should be rounded to 2 decimal places.
Samples
Sample 1
Input
InventoryItem.set_tax_rate(0.05); InventoryItem(200).price_with_tax()
Output
210.0
A 5% tax on 200 yields 210.0.