Router prefix and tags in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the full path of the endpoint?

Given this FastAPI router setup, what is the full URL path to access the read_items endpoint?

from fastapi import APIRouter

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

@router.get("/")
async def read_items():
    return {"msg": "List of items"}

FastAPI

from fastapi import APIRouter

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

@router.get("/")
async def read_items():
    return {"msg": "List of items"}

A /items/read_items

B /read_items/

C /items/

D /

💡 Hint

Remember the prefix adds before the route path.

The router has a prefix '/items' and the endpoint path is '/'. So the full path is '/items/'.

❓ state_output

intermediate

What tags appear in the OpenAPI docs?

Consider this FastAPI router code. Which tags will appear in the OpenAPI documentation for the read_users endpoint?

from fastapi import APIRouter

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

@router.get("/", tags=["custom"])
async def read_users():
    return {"msg": "List of users"}

FastAPI

from fastapi import APIRouter

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

@router.get("/", tags=["custom"])
async def read_users():
    return {"msg": "List of users"}

A ["custom"]

B ["users"]

C ["users", "custom"]

D []

💡 Hint

Endpoint tags override router tags if provided.

When tags are set on the endpoint, they replace the router's tags in the OpenAPI docs.

📝 Syntax

advanced

Identify the syntax error in router prefix usage

Which option contains a syntax error when defining a FastAPI router with a prefix and tags?

Code snippet:

from fastapi import APIRouter

router = APIRouter(prefix=PREFIX, tags=TAG_LIST)

Assume PREFIX and TAG_LIST are variables.

FastAPI

from fastapi import APIRouter

router = APIRouter(prefix=PREFIX, tags=TAG_LIST)

A

PREFIX = "/api"
TAG_LIST = ["api"]

B

PREFIX = "/api"
TAG_LIST = {"api"}

C

PREFIX = "/api"
TAG_LIST = ("api",)

D

PREFIX = "/api"
TAG_LIST = "api"

💡 Hint

Tags must be a list of strings, not a single string.

Tags parameter expects a list of strings. Option D assigns a string instead of a list, causing a runtime error.

🔧 Debug

advanced

Why does the endpoint not appear under the expected tag?

You have this FastAPI router:

from fastapi import APIRouter

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

@router.get("/list", tags=[])
async def list_products():
    return {"msg": "Products list"}

But in the OpenAPI docs, the endpoint appears under the default tag instead of "products". Why?

FastAPI

from fastapi import APIRouter

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

@router.get("/list", tags=[])
async def list_products():
    return {"msg": "Products list"}

A The endpoint has its own tags parameter set to an empty list, overriding router tags.

B The router tags parameter must be a string, not a list.

C The router prefix is incorrect, so tags are ignored.

D The router tags are ignored if the endpoint path starts with a slash.

💡 Hint

Check if the endpoint has tags set explicitly.

If the endpoint has tags set (even empty list), it overrides the router tags in docs. Here, an empty list causes the endpoint to appear under default tag.

🧠 Conceptual

expert

How does router prefix affect nested routers?

Consider two routers in FastAPI:

from fastapi import APIRouter

parent_router = APIRouter(prefix="/parent", tags=["parent"])
child_router = APIRouter(prefix="/child", tags=["child"])

@child_router.get("/endpoint")
async def child_endpoint():
    return {"msg": "Child endpoint"}

parent_router.include_router(child_router)

What is the full path and tags shown in OpenAPI docs for child_endpoint?

FastAPI

from fastapi import APIRouter

parent_router = APIRouter(prefix="/parent", tags=["parent"])
child_router = APIRouter(prefix="/child", tags=["child"])

@child_router.get("/endpoint")
async def child_endpoint():
    return {"msg": "Child endpoint"}

parent_router.include_router(child_router)

A Path: /parent/child/endpoint, Tags: ["parent", "child"]

B Path: /parent/child/endpoint, Tags: ["child"]

C Path: /child/endpoint, Tags: ["child"]

D Path: /parent/endpoint, Tags: ["parent"]

💡 Hint

Prefixes combine when routers are nested; tags do not merge automatically.

The full path combines parent and child prefixes. Tags from child router apply to its endpoints, not parent's tags.

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