How to Create a FastAPI App: Simple Steps to Get Started

To create a FastAPI app, first import FastAPI from the fastapi package, then create an instance with app = FastAPI(). Define routes using decorators like @app.get() to handle HTTP requests and run the app with an ASGI server such as uvicorn.

Syntax

Creating a FastAPI app involves these parts:

• Import FastAPI: Bring in the FastAPI class from the fastapi package.
• Create app instance: Use app = FastAPI() to start your app.
• Define routes: Use decorators like @app.get() or @app.post() to create endpoints that respond to HTTP requests.
• Run the app: Use an ASGI server like uvicorn to serve your app.

python

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def read_root():
    return {"message": "Hello, FastAPI!"}

Example

This example shows a complete FastAPI app that responds with a JSON message at the root URL /. It demonstrates how to create the app, define a GET route, and run it using uvicorn.

python

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def read_root():
    return {"message": "Hello, FastAPI!"}

# To run this app, save as main.py and run:
# uvicorn main:app --reload

Output

{"message": "Hello, FastAPI!"}

Common Pitfalls

Common mistakes when creating a FastAPI app include:

• Not using async def for route functions, which can limit performance.
• Forgetting to create the FastAPI() instance before defining routes.
• Running the app without an ASGI server like uvicorn.
• Using incorrect decorator syntax or missing the route path string.

python

from fastapi import FastAPI

app = FastAPI()

# Wrong: missing async, no parentheses in decorator
# @app.get
# def root():
#     return {"msg": "Hi"}

# Right:
@app.get("/")
async def root():
    return {"msg": "Hi"}

Quick Reference

FastAPI app creation quick tips:

• Import FastAPI and create app = FastAPI().
• Use @app.get("/path") or @app.post("/path") to define routes.
• Define route functions as async def for best performance.
• Run with uvicorn main:app --reload for development.

Key Takeaways

Create a FastAPI app by importing FastAPI and instantiating it with app = FastAPI().

Define routes using decorators like @app.get() with async functions for better performance.

Run your FastAPI app using an ASGI server such as uvicorn for it to work.

Avoid missing parentheses in decorators and always specify route paths as strings.

Use uvicorn with --reload during development to see changes instantly.