Symptom

build_category_filters() creates a dict of filter functions, one per category. But every filter matches the last category in the list, regardless of which key you use.

filters = build_category_filters(["food", "transport", "entertainment"])
food_expense = Expense(10, "food", "Lunch", date(2024, 1, 1))
print(filters["food"](food_expense))        # False -- should be True!
print(filters["entertainment"](food_expense))  # True -- wrong!

What you know

The function loops over a list of category names and creates a lambda for each one. The lambdas are stored in a dictionary keyed by category name. But when you actually call any of them, they all behave as if the category is the last item from the list.