Write the Save Function

Create a function that writes expenses and the counter to a JSON file

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Converting Python data to JSON

Python's json.dumps() function converts a dictionary into a JSON-formatted string. The indent=2 argument adds line breaks and spacing so the file is readable:

json.dumps({"name": "Lunch"}, indent=2)
# '{\n  "name": "Lunch"\n}'

Writing to a file with pathlib

The pathlib module provides Path objects that represent file paths. Path.write_text() writes a string to a file, creating the file if it does not exist:

from pathlib import Path
Path("data.json").write_text("hello")

What to save

You need to save two things: the expenses dictionary and the counter. Without the counter, the server would reset its identifier sequence after a restart. Two expenses could end up with the same identifier.

The save function wraps both values in a single dictionary and writes it to the file.

Instructions

Create the save_expenses function between the models and the expenses = {} line.

Define a function named save_expenses that takes no arguments.
Inside the function, create a dictionary named data with two keys: "counter" set to counter and "expenses" set to expenses.
Call DATA_FILE.write_text(json.dumps(data, indent=2)) to write the data to the file.

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import json
from datetime import datetime
from pathlib import Path
from fastapi import FastAPI, HTTPException
from pydantic import BaseModel, Field, field_validator
from typing import Literal, Optional

app = FastAPI()
DATA_FILE = Path("expenses.json")

class Expense(BaseModel):
    description: str = Field(min_length=1)
    amount: float = Field(gt=0)
    category: Literal["food", "transport", "entertainment", "utilities", "other"]
    date: Optional[str] = None

    @field_validator("date")
    @classmethod
    def validate_date_format(cls, v):
        if v is not None:
            try:
                datetime.strptime(v, "%Y-%m-%d")
            except ValueError:
                raise ValueError("Date must be in YYYY-MM-DD format")
        return v

class ExpenseUpdate(BaseModel):
    description: Optional[str] = Field(default=None, min_length=1)
    amount: Optional[float] = Field(default=None, gt=0)
    category: Optional[Literal["food", "transport", "entertainment", "utilities", "other"]] = None

# Step 1: Define a function named save_expenses
# Step 2: Create data = {"counter": counter, "expenses": expenses}
# Step 3: Write DATA_FILE.write_text(json.dumps(data, indent=2))

expenses = {}
counter = 0

@app.post("/expenses", status_code=201)
def create_expense(expense: Expense):
    global counter
    counter += 1
    expenses[counter] = {"id": counter, **expense.model_dump()}
    return expenses[counter]

@app.get("/expenses")
def list_expenses(category: str = None):
    if category:
        return [e for e in expenses.values() if e["category"] == category]
    return list(expenses.values())

@app.get("/expenses/{expense_id}")
def get_expense(expense_id: int):
    if expense_id not in expenses:
        raise HTTPException(status_code=404, detail="Expense not found")
    return expenses[expense_id]

@app.delete("/expenses/{expense_id}")
def delete_expense(expense_id: int):
    if expense_id not in expenses:
        raise HTTPException(status_code=404, detail="Expense not found")
    return expenses.pop(expense_id)

@app.patch("/expenses/{expense_id}")
def update_expense(expense_id: int, updates: ExpenseUpdate):
    if expense_id not in expenses:
        raise HTTPException(status_code=404, detail="Expense not found")
    expenses[expense_id].update(updates.model_dump(exclude_unset=True))
    return expenses[expense_id]

Write the Save Function

Converting Python data to JSON

Writing to a file with pathlib

What to save

Instructions

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