Lesson 4: Type hints and dataclasses, saying what each value is

Module 9 · Lesson 4 · About 50 minutes · Needs: Module 9 Lessons 1 to 3 (classes, __init__, __repr__, __eq__), Module 6 Lesson 2 (the mutable default trap), Module 7 Lesson 4 (%pip)

The situation

Tom is reading your code on the café laptop. "This late_fee takes days_late. Is that a number? Could it be the text from the spreadsheet? And every class starts with an __init__ that only copies values onto self."

Both are fair. This lesson fixes them: type hints say what each value is, and dataclasses write the repetitive parts of a class for you. Jonah's loan L04, Frankenstein, 16 days late and 300 pence owed today, runs through the whole lesson.