Level 1 / Project 14 - Basic Expense Tracker¶

Learn Your Way¶

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Estimated time: 40 minutes

Focus¶

simple ledger aggregation and reporting

Why this project exists¶

Parse expense records from CSV, group them by category, compute totals and averages, and find the top spenders. You will learn CSV parsing, category aggregation, and sorting with lambda functions.

Run (copy/paste)¶

Use <repo-root> as the folder containing this repository's README.md.

cd <repo-root>/projects/level-1/14-basic-expense-tracker
python project.py --input data/sample_input.txt
pytest -q

Expected terminal output¶

=== Expense Report ===

  Category     Total     Count  Average
  ──────────── ───────── ────── ────────
  Transport    $35.00    1      $35.00
  Food         $12.50    1      $12.50

  Grand total: $47.50
  Top expense: Monthly bus pass ($35.00)
8 passed

Expected artifacts¶

data/output.json
Passing tests
Updated notes.md

Checkpoint: Baseline code runs and all tests pass. Commit your work before continuing.

Alter it (required) — Extension¶

Add a --category filter flag that shows only expenses in a given category.
Add a monthly breakdown showing total spending per month.
Re-run script and tests.

Break it (required) — Core¶

Add a CSV row with a negative amount -- does parse_expense() reject it?
Add a row with a missing category field -- does validation catch it?
Use a file with no data rows (just headers) -- does overall_stats() crash on empty data?

Fix it (required) — Core¶

Ensure parse_expense() rejects negative amounts with a clear ValueError.
Handle the empty-expenses case in overall_stats() by returning zero stats.
Add a test for the missing-field case.

Checkpoint: All modifications done, tests still pass. Good time to review your changes.

Explain it (teach-back)¶

Why does parse_expense() normalise categories to lowercase with .lower()?
What does csv.DictReader do differently from reading lines and splitting on commas?
Why does top_expenses() use sorted() with key=lambda instead of .sort()?
Where would expense tracking appear in real software (personal finance apps, accounting systems, budget reports)?

Mastery check¶

You can move on when you can: - run baseline without docs, - explain one core function line-by-line, - break and recover in one session, - keep tests passing after your change.

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