Real programs read files and survive bad input — here is how to do both cleanly.

Key topics

• The with open(...) as f pattern; read/write/append modes; text vs binary.
• Reading line by line, read() vs readlines(); writing and json/csv module basics.
• Exceptions: try/except/else/finally; catching specific exceptions; raising with raise; custom exception classes.
• EAFP vs LBYL styles; the exception hierarchy; never bare except:.
• Path handling with pathlib.

Exercise / deliverable

Write expenses.py: load a CSV of expenses, skip malformed rows with per-row error messages, compute totals per category, and write a JSON summary — then handle a missing file gracefully.