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.