Python Expense Tracker Project

1Expense Tracker Project: overview

An expense tracker stores each purchase as a dictionary inside a list, adds new entries, totals the spending per category and saves everything to a file so the data survives between runs.

Before this lesson: Python Dictionaries, Python Functions, Python File Handling, Working with JSON in Python

2Expense Tracker Project: diagram

4Expense Tracker Project: syntax

expenses = []                                   # list of dicts
expenses.append({"date": d, "category": c, "amount": a})

totals = {}
for row in expenses:                            # running totals
    totals[row["category"]] = totals.get(row["category"], 0) + row["amount"]

with open("expenses.json", "w") as file:        # save
    json.dump(expenses, file)
with open("expenses.json") as file:             # load
    expenses = json.load(file)

print(f"{category:<10}{amount:>7.2f}")          # aligned report line

5Expense Tracker Project: run the example

Press Run (or Ctrl + Enter). The output (step 6) appears directly below the code. Change anything and run it again.

import json

def add_expense(expenses, date, category, amount):
    expenses.append({"date": date, "category": category, "amount": amount})

def totals_by_category(expenses):
    totals = {}
    for row in expenses:
        totals[row["category"]] = totals.get(row["category"], 0) + row["amount"]
    return totals

expenses = []
add_expense(expenses, "2024-05-01", "food", 12.50)
add_expense(expenses, "2024-05-02", "transport", 3.20)
add_expense(expenses, "2024-05-02", "food", 8.75)
add_expense(expenses, "2024-05-03", "fun", 15.00)

with open("expenses.json", "w") as file:
    json.dump(expenses, file)
with open("expenses.json") as file:
    saved = json.load(file)
totals = totals_by_category(saved)
for category in sorted(totals, key=totals.get, reverse=True):
    print(f"{category:<10}{totals[category]:>7.2f}")
print(f"{'TOTAL':<10}{sum(totals.values()):>7.2f}")

Expected outputfood        21.25
fun         15.00
transport    3.20
TOTAL       39.45

7Expense Tracker Project: how the example works

Hover or tap a step to highlight its lines in the example above.

1. Line 1The json module saves Python lists and dictionaries as text and reads them back.
2. Line 3-4add_expense() builds a dictionary for one expense and appends it to the list it is given. Lists are changed in place, so nothing needs to be returned.
3. Line 6-10totals_by_category() starts with an empty dictionary. For each expense, totals.get(category, 0) fetches the running total (0 for a new category) and the amount is added to it.
4. Line 12-16Start with an empty list and record four expenses. Each one becomes a dictionary like {"date": "2024-05-01", "category": "food", "amount": 12.5}.
5. Line 18-19Open the file in write mode ("w") and json.dump() writes the whole list as JSON text, replacing any older contents.
6. Line 20-21Open it again for reading and json.load() rebuilds the list of dictionaries — exactly what the program would do the next time it starts.
7. Line 22Calculate the totals from the loaded data: food 21.25, transport 3.2 and fun 15.0.
8. Line 23-24sorted(totals, key=totals.get, reverse=True) orders the category names by their totals, biggest first. :<10 pads the name to 10 characters and :>7.2f right-aligns the amount with two decimals.
9. Line 25The grand total is the sum of all category totals, printed in the same column layout.

8Expense Tracker Project: common mistakes

import json
expenses = [{"category": "food", "amount": 4.5}]
for run in range(2):          # the program saves twice
    with open("expenses.json", "a") as file:
        json.dump(expenses, file)
with open("expenses.json") as file:
    data = json.load(file)

import json
expenses = [{"category": "food", "amount": 4.5}]
for run in range(2):
    with open("expenses.json", "w") as file:
        json.dump(expenses, file)
with open("expenses.json") as file:
    data = json.load(file)
print(data)

expenses = [{"category": "food", "amount": 12.5},
            {"category": "food", "amount": 8.75}]
totals = {}
for row in expenses:
    totals[row["category"]] = row["amount"]
print(totals)

expenses = [{"category": "food", "amount": 12.5},
            {"category": "food", "amount": 8.75}]
totals = {}
for row in expenses:
    totals[row["category"]] = totals.get(row["category"], 0) + row["amount"]
print(totals)

amount = input("Amount: ")
if amount <= 0:
    print("Amount must be positive")

amount = float(input("Amount: "))
if amount <= 0:
    print("Amount must be positive")

9Expense Tracker Project: exercises

Write your answer, then press Check. The checks run in your browser.

expenses = [
    {"category": "food", "amount": 4.5},
    {"category": "books", "amount": 12.0},
    {"category": "food", "amount": 3.0},
]
totals = {}
for e in expenses:
    totals[e["category"]] = totals.get(e["category"], 0) + e["amount"]
print(totals)
print(len(expenses), "expenses")
print(f"food: {totals['food']:.2f}")

{'food': 7.5, 'books': 12.0}
3 expenses
food: 7.50

def add_expense(expenses, date, category, amount):
    expenses.append({"date": date, "category": category, "amount": amount})

expenses = []
add_expense(expenses, "2024-05-01", " Food ", 12.5)
print(expenses)

def add_expense(expenses, date, category, amount):
    category = category.strip().lower()
    if not category:
        raise ValueError("category cannot be empty")
    if amount <= 0:
        raise ValueError("amount must be greater than 0")
    expenses.append({"date": date, "category": category, "amount": amount})

expenses = []
add_expense(expenses, "2024-05-01", " Food ", 12.5)
print(expenses)

2 more exercises → 4 more examples

Further reading

• Python tutorial

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