Including routers in main app in FastAPI - Step-by-Step Execution

Concept Flow - Including routers in main app

Define Router

↓

Add Routes to Router

↓

Create Main FastAPI App

↓

Include Router in Main App

↓

Run App and Handle Requests

This flow shows how you define a router, add routes to it, create the main app, include the router, and then run the app to handle requests.

Execution Sample

FastAPI

from fastapi import FastAPI, APIRouter

router = APIRouter()

@router.get('/hello')
async def say_hello():
    return {'message': 'Hello from router!'}

app = FastAPI()
app.include_router(router, prefix='/api')

This code creates a router with one GET route, then includes it in the main FastAPI app under the '/api' prefix.

Execution Table

Step	Action	Object Created/Modified	Effect
1	Import FastAPI and APIRouter	Modules imported	Ready to use FastAPI and router classes
2	Create APIRouter instance	router	Router object created to hold routes
3	Define GET route '/hello' on router	router with route	Route '/hello' added to router
4	Create FastAPI app instance	app	Main app object created
5	Include router in app with prefix '/api'	app with router	Router routes accessible under '/api' prefix
6	Run app and receive GET request to '/api/hello'	Request handled	Returns JSON {'message': 'Hello from router!'}
7	Request to '/hello' (without prefix)	No route found	404 Not Found error
8	Request to '/api/unknown'	No route found	404 Not Found error

💡 Execution stops after handling requests or when no matching route is found.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
router	None	APIRouter instance	APIRouter with '/hello' route	APIRouter with '/hello' route	Same	Same
app	None	None	None	FastAPI instance	FastAPI instance with router included	Same

Key Moments - 3 Insights

Why does the route '/hello' not work directly on the main app?

Because the route is defined on the router and included with the prefix '/api', the full path becomes '/api/hello'. Requests to '/hello' do not match any route as shown in execution_table row 7.

What happens if you include the router without a prefix?

The router's routes become available at their original paths. For example, '/hello' would work directly. This is implied by the prefix usage in step 5 and the 404 in step 7.

Can you include multiple routers in the main app?

Yes, you can create multiple APIRouter instances and include each with different prefixes or none. This modularizes your app routes.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'router' after step 3?

A An APIRouter instance with the '/hello' route added

B None

C A FastAPI app instance

D An empty APIRouter instance

💡 Hint

Check the 'router' variable in variable_tracker after step 3 and execution_table row 3.

At which step does the main app include the router?

A Step 2

B Step 5

C Step 4

D Step 6

💡 Hint

Look for the action 'Include router in app' in execution_table.

If you remove the prefix '/api' when including the router, what changes in the execution?

A The router is not included at all

B The route '/api/hello' still works

C The route '/hello' is accessible directly on the main app

D The app crashes

💡 Hint

Refer to key_moments about prefix usage and execution_table rows 6 and 7.

Concept Snapshot

Define APIRouter and add routes to it
Create FastAPI app instance
Include router in app with app.include_router(router, prefix)
Routes on router are accessed with prefix + route path
Without prefix, routes are at root paths
Use routers to organize app routes modularly

Full Transcript

This example shows how to organize FastAPI routes using routers. First, you import FastAPI and APIRouter. Then, create a router instance and add routes to it, like a GET route at '/hello'. Next, create the main FastAPI app instance. Include the router in the main app using app.include_router(router, prefix='/api'). This means the router's routes are accessed under the '/api' path, so '/api/hello' works but '/hello' does not. When the app runs, requests to '/api/hello' return the expected JSON message. Requests to paths not defined return 404 errors. Using routers helps keep your app organized by grouping related routes and optionally adding prefixes.