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.

Practice

1. What is the main purpose of using a prefix in a FastAPI router?

easy

A. To add authentication to routes automatically

B. To group routes under a common URL path segment

C. To change the HTTP method of all routes

D. To disable routes temporarily

Solution

1. Step 1: Understand what prefix does in FastAPI routers

   The prefix adds a common path segment to all routes defined in that router, grouping them logically.

2. Step 2: Compare options with prefix purpose

   Only To group routes under a common URL path segment correctly describes grouping routes under a shared URL path segment.

3. Final Answer:

   To group routes under a common URL path segment -> Option B

4. Quick Check:

   Router prefix = group routes [OK]

Hint: Prefix groups routes by URL path segment [OK]

Common Mistakes:

• Thinking prefix changes HTTP methods
• Confusing prefix with authentication
• Assuming prefix disables routes

2. Which of the following is the correct way to add a prefix and tags to a FastAPI router?

easy

A. router = APIRouter(prefix='/items', tags=['items'])

B. router = APIRouter(tags='/items', prefix=['items'])

C. router = APIRouter(prefix=['items'], tags='/items')

D. router = APIRouter(prefix='/items', tags='/items')

Solution

1. Step 1: Recall correct parameter types for prefix and tags

   Prefix expects a string starting with '/', tags expects a list of strings.

2. Step 2: Check each option's syntax

   router = APIRouter(prefix='/items', tags=['items']) uses prefix as string and tags as list correctly; others mix types incorrectly.

3. Final Answer:

   router = APIRouter(prefix='/items', tags=['items']) -> Option A

4. Quick Check:

   Prefix string + tags list = correct syntax [OK]

Hint: Prefix is string, tags is list of strings [OK]

Common Mistakes:

• Using list for prefix instead of string
• Using string for tags instead of list
• Mixing order of parameters incorrectly

3. Given this code snippet, what is the full path of the endpoint?

from fastapi import APIRouter

router = APIRouter(prefix='/users', tags=['users'])

@router.get('/profile')
async def get_profile():
    return {'msg': 'User profile'}

medium

A. /profile/users

B. /profile

C. /users

D. /users/profile

Solution

1. Step 1: Identify router prefix and route path

   The router has prefix '/users' and the route path is '/profile'.

2. Step 2: Combine prefix and route path

   Full path is prefix + route path = '/users' + '/profile' = '/users/profile'.

3. Final Answer:

   /users/profile -> Option D

4. Quick Check:

   Prefix + route path = full endpoint path [OK]

Hint: Full path = prefix + route path [OK]

Common Mistakes:

• Ignoring prefix and using only route path
• Swapping prefix and route path order
• Confusing tags with path

4. What is wrong with this FastAPI router code?

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

medium

A. prefix should be a list, tags should be a string

B. Both prefix and tags should be strings

C. prefix should be a string, tags should be a list of strings

D. No error, this is valid syntax

Solution

1. Step 1: Check expected types for prefix and tags

   Prefix must be a string starting with '/', tags must be a list of strings.

2. Step 2: Analyze given code

   Prefix is given as a list, tags as a string, both reversed from expected types.

3. Final Answer:

   prefix should be a string, tags should be a list of strings -> Option C

4. Quick Check:

   Prefix = string, tags = list [OK]

Hint: Prefix string, tags list; reverse causes error [OK]

Common Mistakes:

• Using list for prefix
• Using string for tags
• Assuming both can be strings

5. You want to create two routers in FastAPI: one for /products tagged 'products' and another for /orders tagged 'orders'. How do you correctly set prefixes and tags to keep your API organized?

hard

A. router_products = APIRouter(prefix='/products', tags=['products'])\nrouter_orders = APIRouter(prefix='/orders', tags=['orders'])

B. router_products = APIRouter(prefix=['products'], tags='products')\nrouter_orders = APIRouter(prefix=['orders'], tags='orders')

C. router_products = APIRouter(prefix='/products', tags='products')\nrouter_orders = APIRouter(prefix='/orders', tags='orders')

D. router_products = APIRouter(tags=['products'])\nrouter_orders = APIRouter(prefix='/orders')

Solution

1. Step 1: Recall correct types for prefix and tags

   Prefix must be a string starting with '/', tags must be a list of strings.

2. Step 2: Evaluate each option for correctness and organization

   router_products = APIRouter(prefix='/products', tags=['products'])\nrouter_orders = APIRouter(prefix='/orders', tags=['orders']) correctly assigns string prefixes and list tags for both routers, organizing API paths and documentation clearly.

3. Final Answer:

   router_products = APIRouter(prefix='/products', tags=['products']) router_orders = APIRouter(prefix='/orders', tags=['orders']) -> Option A

4. Quick Check:

   Correct prefix and tags keep API organized [OK]

Hint: Use string prefix and list tags for each router [OK]

Common Mistakes:

• Using list for prefix or string for tags
• Omitting prefix or tags on one router
• Mixing tags and prefix types