Route ordering and priority in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Route Matching Priority in FastAPI

Given these two routes in a FastAPI app, which route will handle a GET request to /items/42?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get('/items/{item_id}')
async def read_item(item_id: int):
    return {"route": "item_id route", "id": item_id}

@app.get('/items/special')
async def read_special():
    return {"route": "special route"}

A FastAPI raises a runtime error due to ambiguous routes.

B The route '/items/special' handles the request.

C Both routes handle the request simultaneously.

D The route '/items/{item_id}' handles the request.

💡 Hint

Consider how FastAPI matches dynamic path parameters versus static paths.

FastAPI matches routes in the order they are defined. The request '/items/42' matches the first route '/items/{item_id}', so the 'read_item' function handles it.

📝 Syntax

intermediate

Correct Route Declaration for Overlapping Paths

Which option correctly declares two FastAPI routes so that /users/me returns a special response and /users/{user_id} returns a user by ID without conflict?

A

@app.get('/users/me')
async def get_me():
    return {"user": "current user"}

@app.get('/users/{user_id}')
async def get_user(user_id: str):
    return {"user_id": user_id}

B

@app.get('/users/{user_id}')
async def get_user(user_id: str):
    return {"user_id": user_id}

@app.get('/users/me')
async def get_me():
    return {"user": "current user"}

C

@app.get('/users/{user_id}')
async def get_user(user_id: str):
    return {"user_id": user_id}

@app.get('/users/{me}')
async def get_me():
    return {"user": "current user"}

D

@app.get('/users/me')
async def get_me():
    return {"user": "current user"}

@app.get('/users/{id}')
async def get_user(id: str):
    return {"user_id": id}

💡 Hint

Static routes should be declared before dynamic routes to avoid conflicts.

FastAPI matches routes in the order they are declared. Declaring the static route '/users/me' before the dynamic '/users/{user_id}' ensures '/users/me' is matched first and does not get captured by the dynamic route.

🔧 Debug

advanced

Unexpected Route Matching Behavior

A FastAPI app has these routes declared in this order. A GET request to /files/download returns the response from which route?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get('/files/{file_name}')
async def get_file(file_name: str):
    return {"file": file_name}

@app.get('/files/download')
async def download_file():
    return {"action": "download"}

A The request results in a 404 Not Found error.

B The '/files/{file_name}' route handles the request with file_name='download'.

C FastAPI raises a runtime error due to route conflict.

D The '/files/download' route handles the request.

💡 Hint

Check the order of route declarations and how FastAPI matches dynamic routes.

FastAPI matches routes in the order they are declared. Since '/files/{file_name}' is declared before '/files/download', the request '/files/download' matches the dynamic route first, treating 'download' as the file_name parameter.

❓ state_output

advanced

Route Response Based on Declaration Order

Consider this FastAPI app. What JSON response will a GET request to /products/latest return?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get('/products/{product_id}')
async def get_product(product_id: str):
    return {"product_id": product_id}

@app.get('/products/latest')
async def get_latest_product():
    return {"product": "latest"}

A {"product_id": "latest"}

B {"product": "latest"}

C 404 Not Found error

D 500 Internal Server Error

💡 Hint

Route order affects which handler is called for overlapping paths.

Since '/products/{product_id}' is declared before '/products/latest', the request '/products/latest' matches the dynamic route first, returning the product_id as 'latest'.

🧠 Conceptual

expert

Ensuring Static Routes Take Priority Over Dynamic Ones

In FastAPI, how can you guarantee that a static route like /api/status is always matched before a dynamic route like /api/{param} regardless of their declaration order?

A Use the <code>include_in_schema=False</code> parameter on the dynamic route.

B Use the <code>priority</code> parameter in the route decorator to set static routes higher.

C Use a custom APIRouter with a route matcher that prioritizes static routes.

D Declare the static route before the dynamic route in the code.

💡 Hint

FastAPI does not have a built-in priority parameter; think about extending routing behavior.

FastAPI matches routes in the order they are declared. To guarantee static routes always take priority regardless of order, you must customize the routing logic, for example by using a custom APIRouter with a matcher that prioritizes static routes over dynamic ones.