Build a REST API with FastAPI

Build a personal expense tracker API with endpoints to add, list, filter, and delete expenses — persisted to a JSON file.

Intermediate

4 hours

7 lessons

Course Content

33 chapters across 7 lessons

How APIs Work

Learn the request/response model, install FastAPI, and define a validated data model

6 chapters

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What Is a REST API?

Continue

Understand the request/response cycle and HTTP methods

Install FastAPI

Install FastAPI, uvicorn, and httpx in your terminal

What Is Pydantic?

Understand why raw JSON needs validation before your code can trust it

Create the Expense Model

Define a Pydantic model with field-level validation for expenses

Validate the Date Format

Add a custom validator that checks the date field is in YYYY-MM-DD format

Lesson Complete

You finished How APIs Work

Create an Expense

Build a POST endpoint that accepts, stores, and returns expenses

5 chapters

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How POST Requests Work

Understand how POST sends data in the request body as JSON

Write the POST Endpoint

Define the POST route and function that receives an Expense

Store and Return the Expense

Assign an identifier, store the expense, and return it to the client

Test with Swagger

Start the server and test your POST endpoint in the browser

Lesson Complete

You finished Create an Expense

List and Filter Expenses

Add a GET endpoint that returns all expenses with an optional category filter

4 chapters

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How Query Parameters Work

Understand how query parameters pass extra information in a URL

List All Expenses

Add a GET endpoint that returns all stored expenses

Filter by Category

Add an optional category query parameter to filter expenses

Lesson Complete

You finished List and Filter Expenses

Get and Delete an Expense

Retrieve a single expense by its identifier and remove expenses from the tracker

4 chapters

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Path Parameters and Error Handling

Understand how FastAPI extracts values from the URL and how to return proper error responses

Get Expense by ID

Add an endpoint that retrieves a single expense using a path parameter

Delete Expense by ID

Add an endpoint that removes an expense from the tracker

Lesson Complete

You finished Get and Delete an Expense

Update an Expense

Add a PATCH endpoint that applies partial updates to existing expenses

4 chapters

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Partial Updates with PATCH

Understand the difference between PUT and PATCH and how exclude_unset works

The ExpenseUpdate Model

Create a Pydantic model where every field is optional for partial updates

Write the PATCH Endpoint

Add a PATCH route that applies partial updates to a stored expense

Lesson Complete

You finished Update an Expense

Save Your Data

Persist expenses to a JSON file so data survives server restarts

5 chapters

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Why Persistence Matters

Understand why in-memory data disappears and how file-based storage fixes it

Write the Save Function

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

Write the Load Function

Create a function that reads expenses from the JSON file on startup

Wire Up Save and Load

Call load on startup and save after every change to persist expenses

Lesson Complete

You finished Save Your Data

Polish Your API

Add response models, error documentation, and a spending summary endpoint

5 chapters

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Why Response Models Matter

Understand how response models define a clear contract between your API and its clients

Add Response Models

Create ExpenseOut and ErrorResponse models and attach them to your endpoints

Document Error Responses

Add response models and error documentation to the get, delete, and update endpoints

Spending Summary

Build a capstone endpoint that calculates total spending per category

Lesson Complete

You finished Polish Your API