Router prefix and tags in FastAPI

Introduction

Router prefix helps group related routes under a common path. Tags organize routes in the API docs for easy reading.

When you want to group user-related routes under '/users' path.

When you want to separate admin routes from public routes.

When you want to add descriptive tags to routes for better API documentation.

When building a large API with many endpoints to keep it organized.

When you want to reuse a router with a different prefix in multiple places.

Syntax

FastAPI

from fastapi import APIRouter

router = APIRouter(prefix="/prefix_path", tags=["tag1", "tag2"])

@router.get("/endpoint")
async def example():
    return {"message": "Hello"}

The prefix adds a common path before all routes in this router.

The tags list adds labels to routes for API docs grouping.

Examples

This groups user routes under '/users' and tags them as 'Users' in docs.

FastAPI

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

@router.get("/")
async def get_users():
    return ["Alice", "Bob"]

This router has two tags and a prefix for item-related routes.

FastAPI

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

@router.get("/{item_id}")
async def read_item(item_id: int):
    return {"item_id": item_id}

You can also add tags directly on route decorators without prefix.

FastAPI

router = APIRouter()

@router.get("/status", tags=["Health"])
async def status():
    return {"status": "ok"}

Sample Program

This FastAPI app has two routers. One for users with prefix '/users' and tag 'Users'. Another for items with prefix '/items' and tag 'Items'. This organizes routes and API docs clearly.

FastAPI

from fastapi import FastAPI, APIRouter

app = FastAPI()

user_router = APIRouter(prefix="/users", tags=["Users"])

@user_router.get("/")
async def list_users():
    return ["Alice", "Bob"]

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

@item_router.get("/{item_id}")
async def get_item(item_id: int):
    return {"item_id": item_id, "name": "Sample Item"}

app.include_router(user_router)
app.include_router(item_router)

Important Notes

Router prefixes help avoid repeating common path parts in every route.

Tags improve the auto-generated API docs readability by grouping endpoints.

You can add tags either on the router or on individual routes.

Summary

Router prefix groups routes under a shared path.

Tags label routes for better API documentation.

Using both keeps your API organized and easy to understand.

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