Manage Settings and Environment Variables in FastAPI

Introduction

FastAPI is a modern, fast (high-performance) web framework for building APIs with Python. When building real-world applications, you’ll often need to manage sensitive information like API keys, database URLs, or feature toggles that should vary across environments (development, testing, production). Using environment variables and Pydantic’s BaseSettings is a recommended and elegant way to manage these configurations in FastAPI.

In this tutorial, you’ll learn how to create a settings class using Pydantic, load environment variables from a .env file, and integrate these settings into your FastAPI application.

💡 Why use Pydantic BaseSettings in FastAPI?

Pydantic’s BaseSettings offers:
✅ Automatic environment variable loading: Reads variables from your environment or a .env file at runtime.
✅ Type validation: Ensures values match your expected types (e.g. str, bool, int).
✅ Flexibility: Easily override settings without changing code.
✅ Security: Keeps secrets out of source code.

✅ Prerequisites

Before you start, make sure you have:

• Python 3.10+ installed
• A working Python environment

📦1️⃣ Install Libraries

First, install the required packages:

pip install fastapi uvicorn pydantic-settings

🔑2️⃣ Create a .env File

In the root of your project, create a file named .env with the following content:

APP_NAME=FastAPI Sample App
ENVIRONMENT=development
ADMIN_USER=admin@example.com

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⚙️3️⃣ Define Your Settings Class

Create a file named settings.py and define your settings using Pydantic’s BaseSettings: