Use Settings in Your App

Replace hardcoded values with settings from your configuration class

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From hardcoded to configurable

You created a Settings class with data_file and app_title fields. Now you need to use those values instead of the hardcoded ones. Two lines in your code need to change:

DATA_FILE = Path("expenses.json") should read from settings.data_file
app = FastAPI() should use settings.app_title as the title

After this change, you can control both values through environment variables or a .env file without touching the code.

The app title in Swagger

The title parameter in FastAPI() sets the heading that appears at the top of the Swagger documentation page at /docs. Right now it shows the default "FastAPI". Setting it to a descriptive name like "Expense Tracker API" makes the documentation easier to identify.

Instructions

Replace the hardcoded values with your settings.

Change DATA_FILE = Path("expenses.json") to DATA_FILE = Path(settings.data_file).
Change app = FastAPI() to app = FastAPI(title=settings.app_title).

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import json
import os
import time
from datetime import datetime
from pathlib import Path
from fastapi import FastAPI, HTTPException
from fastapi.middleware.cors import CORSMiddleware
from pydantic import BaseModel, Field, field_validator
from pydantic_settings import BaseSettings, SettingsConfigDict
from typing import Literal, Optional

# Step 2: Change FastAPI() to use settings.app_title
app = FastAPI()
app.add_middleware(CORSMiddleware, allow_origins=["*"], allow_methods=["*"], allow_headers=["*"])

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

class ExpenseOut(BaseModel):
    id: int
    description: str
    amount: float
    category: str
    date: Optional[str] = None

class ErrorResponse(BaseModel):
    detail: str

class Settings(BaseSettings):
    data_file: str = "expenses.json"
    app_title: str = "FastAPI"
    model_config = SettingsConfigDict(env_file=".env")

settings = Settings()

# Step 1: Change DATA_FILE to use settings.data_file
DATA_FILE = Path("expenses.json")

def save_expenses():
    temp_path = str(DATA_FILE) + ".tmp"
    with open(temp_path, "w") as f:
        json.dump({"counter": counter, "expenses": expenses}, f, indent=2)
    os.replace(temp_path, str(DATA_FILE))

def load_expenses():
    global expenses, counter
    if DATA_FILE.exists():
        data = json.loads(DATA_FILE.read_text())
        expenses = {int(k): v for k, v in data["expenses"].items()}
        counter = data["counter"]

expenses = {}
counter = 0
load_expenses()

@app.middleware("http")
async def log_requests(request, call_next):
    start = time.time()
    response = await call_next(request)
    duration = (time.time() - start) * 1000
    print(f"{request.method} {request.url.path} {response.status_code} {duration:.1f}ms")
    return response

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

@app.get("/expenses", response_model=list[ExpenseOut])
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}", response_model=ExpenseOut, responses={404: {"model": ErrorResponse}})
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}", response_model=ExpenseOut, responses={404: {"model": ErrorResponse}})
def delete_expense(expense_id: int):
    if expense_id not in expenses:
        raise HTTPException(status_code=404, detail="Expense not found")
    save_expenses()
    return expenses.pop(expense_id)

@app.patch("/expenses/{expense_id}", response_model=ExpenseOut, responses={404: {"model": ErrorResponse}})
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))
    save_expenses()
    return expenses[expense_id]

@app.get("/summary")
def spending_summary():
    summary = {}
    for expense in expenses.values():
        cat = expense["category"]
        summary[cat] = summary.get(cat, 0) + expense["amount"]
    return summary

Use Settings in Your App

From hardcoded to configurable

The app title in Swagger

Instructions

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