Project: An Expense Tracker

Why this matters

Time for a second project. This expense tracker is a small command-line program that reads commands, rejects bad input politely, stores data in a file and prints a report. It brings together exactly what this module taught: catching errors, raising your own, and keeping a program running when things go wrong.

The idea

The main program is already written at the bottom of the starter. You write the five functions it relies on:

Function	Job
parse_expense(line)	turn "food 12.50" into ("food", 12.5) or raise ValueError
load_expenses(path)	read saved expenses; no file yet means an empty list
save_expense(path, category, amount)	append one line to the file
totals_by_category(expenses)	add up the amounts for each category
format_report(totals)	build the printed report

Each stored line looks like food,12.5. The main program turns your ValueError into a friendly Error: ... line instead of crashing.

Try it

Read the main() function first. Trace what happens for the commands add food 12.5, then add food, then report. Which of your functions runs each time?

Your turn

Complete the five functions. The error messages and layouts below are exact, because the checks compare them.

parse_expense splits the line on spaces. It lower-cases the category and returns the amount as a float. It raises ValueError with exactly these messages:

• wrong number of parts: expected CATEGORY AMOUNT, for example: food 12.50
• amount is not a number: amount must be a number
• amount is zero or less: amount must be positive

load_expenses returns a list of (category, amount) tuples. A missing file gives []. Blank or broken lines are skipped.

totals_by_category returns a dictionary of category to total, each rounded to 2 decimals.

format_report returns No expenses yet. for an empty dictionary. Otherwise one line per category, biggest amount first (ties by name), then a total line. Each line is the category padded to 10 characters followed by the amount right-aligned in 8 characters with 2 decimals:

rent        500.00
food         12.50
total       512.50

Recap

• Validate input in one place and raise clear errors; handle them in the caller.
• A missing file is often a normal situation, not a crash.
• Small functions that each do one job are easy to test and reuse.

Common mistakes

• Catching Exception in main instead of the specific ValueError, which would hide real bugs.
• Opening the save file with "w", which wipes earlier expenses. Use "a" to append.
• Forgetting to lower-case the category, so Food and food are totalled separately.