Router prefix and tags in FastAPI - Step-by-Step Execution

Concept Flow - Router prefix and tags

Create APIRouter

↓

Set prefix and tags

↓

Define route functions

↓

Include router in main app

↓

Client sends request

↓

Match prefix + path

↓

Execute route function

↓

Return response with tags metadata

This flow shows how a router with a prefix and tags is created, routes are defined, and requests are matched using the prefix before executing the route.

Execution Sample

FastAPI

from fastapi import FastAPI, APIRouter

router = APIRouter(prefix="/items", tags=["items"])

@router.get("/")
async def read_items():
    return [{"item_id": "foo"}]

app = FastAPI()
app.include_router(router)

Defines a router with prefix '/items' and tag 'items', then includes it in the main FastAPI app.

Execution Table

Step	Action	Prefix	Tags	Route Path	Request URL	Matched Route	Response
1	Create APIRouter	/items	["items"]	N/A	N/A	N/A	N/A
2	Define route @router.get("/")	/items	["items"]	/	N/A	N/A	N/A
3	Include router in app	/items	["items"]	/	N/A	N/A	N/A
4	Client sends GET request	/items	["items"]	/	/items/	Yes	[{"item_id": "foo"}]
5	Client sends GET request	/items	["items"]	/	/items/123	No	404 Not Found
6	Client sends GET request	/items	["items"]	/	/other/	No	404 Not Found

💡 Execution stops after matching request URL with router prefix and route path or returning 404 if no match.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
router.prefix	None	/items	/items	/items	/items
router.tags	None	["items"]	["items"]	["items"]	["items"]
route.path	None	N/A	/	/	/
app.routers	[]	[]	[]	[router]	[router]

Key Moments - 3 Insights

Why does the route path '/' become '/items/' when accessed?

Because the router has a prefix '/items', all routes inside it are accessed with that prefix added before the route path. See execution_table step 4.

What happens if a request URL does not start with the router prefix?

The router does not match the request, so FastAPI returns a 404 Not Found. See execution_table steps 5 and 6.

How do tags affect the API?

Tags group routes in the automatic API docs for easier navigation. They do not affect routing behavior. This is set when creating the router (step 1).

Visual Quiz - 3 Questions

Look at the execution_table at step 4. What is the response when the client requests '/items/'?

A [{"item_id": "foo"}]

B 404 Not Found

C Empty list []

D Error: No route

💡 Hint

Check the 'Response' column at step 4 in execution_table.

At which step does the router get included in the main FastAPI app?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column for including router in app.

If the router prefix was changed to '/products', what would be the matched route for request URL '/items/'?

A Matched route with response [{"item_id": "foo"}]

B No match, 404 Not Found

C Matched route but empty response

D Server error

💡 Hint

Refer to how prefix affects matching in execution_table steps 4 and 5.

Concept Snapshot

APIRouter(prefix, tags) sets a common path prefix and groups routes.
Routes inside use relative paths.
Include router in FastAPI app to activate.
Requests match prefix + route path.
Tags organize routes in docs, no routing effect.

Full Transcript

This visual execution shows how FastAPI's APIRouter uses prefix and tags. First, a router is created with a prefix '/items' and tags ['items']. Then a route is defined at path '/'. When the router is included in the main app, all routes inside it are accessed with the prefix added, so '/' becomes '/items/'. When a client sends a request to '/items/', the router matches and executes the route function, returning the response. Requests not starting with the prefix do not match and return 404. Tags help organize routes in API docs but do not affect routing. The variable tracker shows how prefix and tags stay constant after setup. Key moments clarify why prefix changes route paths and how unmatched URLs behave. The quiz tests understanding of routing and prefix effects.