Router prefix and tags in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a prefix in a FastAPI router?

The prefix adds a common path to all routes in the router, so you don't have to repeat it for each route. It helps organize routes under a shared URL segment.

beginner

How do tags help when using routers in FastAPI?

tags group routes in the automatic API docs, making it easier to find and understand related endpoints.

beginner

Show an example of creating a FastAPI router with a prefix /items and tag Items.

from fastapi import APIRouter

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

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

intermediate

What happens if you add a prefix to a router and then include it in the main FastAPI app?

All routes in that router will have the prefix added to their paths automatically when included in the main app.

beginner

Can you assign multiple tags to a FastAPI router? How?

Yes, by passing a list of strings to the tags parameter, e.g., tags=["Users", "Admin"].

What does the prefix parameter do in a FastAPI router?

A Changes the HTTP method of routes

B Groups routes in the API docs

C Adds a common path to all routes in the router

D Defines the response model for routes

The prefix adds a shared path segment to all routes in the router.

How do tags affect FastAPI routes?

A They define middleware for routes

B They change the URL path of routes

C They set the HTTP status code

D They group routes in the API documentation

Tags group routes in the automatic API docs for better organization.

If a router has prefix /users and a route @router.get("/profile"), what is the full path?

A /users/profile

B /profile/users

C /users

D /profile

The prefix is added before the route path, so the full path is /users/profile.

Which of these is a valid way to assign multiple tags to a FastAPI router?

A tags="Users,Admin"

B tags=["Users", "Admin"]

C tags=(Users, Admin)

D tags={Users, Admin}

Tags must be a list of strings, like ["Users", "Admin"].

Where do you define the prefix and tags for a group of routes in FastAPI?

A When creating the APIRouter instance

B Inside each route function

C In the main FastAPI app only

D In the response model

Prefix and tags are set when creating the APIRouter instance.

Explain how router prefixes and tags help organize routes in a FastAPI application.

Think about URL paths and API documentation.

Describe how you would create a router with prefix '/products' and tags ['Products', 'Store'] in FastAPI.

Focus on APIRouter parameters.

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