How to Create Routes in FastAPI: Simple Guide

In FastAPI, you create a route by defining a path operation function decorated with @app.get(), @app.post(), or other HTTP method decorators on an FastAPI app instance. The decorator specifies the URL path, and the function defines the response for that route.

Syntax

To create a route in FastAPI, you use a decorator on a function that handles requests. The decorator defines the HTTP method and the URL path. The function returns the response data.

• @app.get(path): Handles GET requests to the given path.
• @app.post(path): Handles POST requests.
• path: The URL path as a string, e.g., "/items/{item_id}".
• function: The function runs when the route is accessed and returns the response.

python

from fastapi import FastAPI

app = FastAPI()

@app.get("/path")
def function_name():
    return {"message": "response"}

Example

This example shows how to create a simple GET route at /hello that returns a greeting message as JSON.

python

from fastapi import FastAPI

app = FastAPI()

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

Output

When you run this FastAPI app and visit http://localhost:8000/hello in a browser or API client, you get the JSON response: {"message": "Hello, FastAPI!"}

Common Pitfalls

Common mistakes when creating routes in FastAPI include:

• Forgetting to create the FastAPI() app instance before defining routes.
• Using the wrong decorator for the HTTP method (e.g., using @app.get for POST requests).
• Not including the path string in the decorator.
• Defining functions without returning a valid response (like a dict or Pydantic model).

Example of a wrong and right way:

python

from fastapi import FastAPI

app = FastAPI()

# Wrong: missing path string
# @app.get()
def no_path():
    return {"error": "No path defined"}

# Right:
@app.get("/correct")
def correct_path():
    return {"message": "This works"}

Quick Reference

Decorator	Purpose	Example
@app.get(path)	Handles GET requests	@app.get("/items")
@app.post(path)	Handles POST requests	@app.post("/items")
@app.put(path)	Handles PUT requests	@app.put("/items/{id}")
@app.delete(path)	Handles DELETE requests	@app.delete("/items/{id}")

Key Takeaways

Create routes by decorating functions with HTTP method decorators on a FastAPI app instance.

Always specify the URL path as a string in the decorator.

Return JSON-serializable data like dicts from route functions.

Use the correct decorator matching the HTTP method you want to handle.

Initialize your FastAPI app before defining routes.