FastAPI – Simple Route Example

This document demonstrates how to create a basic FastAPI application with a single HTTP route and how to run it using uvicorn, which is the default and recommended ASGI server for FastAPI.

Create a Simple FastAPI Application

Create a Python file named main.py and add the following content:

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def home_dir():
    return {"message": "Home Page"}

Explanation

FastAPI() initializes the application instance.
@app.get("/") registers an HTTP GET endpoint at the root path (/).
The home_dir function is the request handler and returns a JSON response.
FastAPI automatically handles JSON serialization and response headers.

Running the Application

FastAPI applications are typically run using uvicorn, an ASGI server designed for high performance.

Option 1: Run Using FastAPI CLI (Development Mode)

FastAPI provides a development-friendly CLI wrapper that uses uvicorn internally:

fastapi dev main.py

This command:

Starts the application in development mode
Enables auto-reload on code changes
Automatically binds to a local development interface

Option 2: Run Directly with Uvicorn (Recommended)

For explicit control over runtime configuration, run the application directly with uvicorn:

uvicorn main:app --reload --host 0.0.0.0 --port 1234

Command Breakdown

main → Python file name (without .py)
app → FastAPI application instance
--reload → Automatically reloads the server on code changes (development only)
--host 0.0.0.0 → Exposes the service on all network interfaces
--port 1234 → Custom application port

Accessing the Application

Once running, the application will be available at:

API Endpoint: http://localhost:1234/
Interactive API Docs (Swagger UI): http://localhost:1234/docs
Alternative API Docs (ReDoc): http://localhost:1234/redoc

Best Practices

Use --reload only in development environments
In production, run uvicorn behind a process manager (e.g., systemd, Docker, Kubernetes)
Explicitly define host and port for containerized and cloud deployments
Keep the application entry point (main:app) consistent across environments

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