Lesson 3: Context managers, clean-up that always happens

Module 10 · Lesson 3 · About 45 minutes · Needs: Module 8 (files with encoding="utf-8", try, except, finally), Module 9 Lessons 1 and 2 (classes, inheritance), Lessons 1 and 2 of this module (yield, the @ line)

The situation

Amira's request for this module is "Make sure it never breaks." Saving is where it breaks worst. Your program writes data/loans.csv one line at a time. If it stops half way, through a bug or a closed laptop, the file holds only the lines written so far.

This lesson follows one save of the 40 loans, crashing on purpose before line 22. A plain save loses 20 loans. By the end, a safe save loses none.