Shared dependencies across routers in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the FastAPI class.

FastAPI

from fastapi import [1]
app = [1]()

Drag options to blanks, or click blank then click option'

AAPIRouter

BDepends

CFastAPI

DRequest

Attempts:

3 left

💡 Hint

Common Mistakes

Importing Depends instead of FastAPI

Using APIRouter to create the main app

The FastAPI class is imported from fastapi to create the app instance.

2fill in blank

medium

Complete the code to define a dependency function that returns a string.

FastAPI

def common_dependency():
    return [1]

Drag options to blanks, or click blank then click option'

A'shared value'

BTrue

CNone

D42

Attempts:

3 left

💡 Hint

Common Mistakes

Returning a number instead of a string

Returning None which is not useful here

The dependency returns a string 'shared value' to be used in routes.

3fill in blank

hard

Fix the error in the router dependency declaration.

FastAPI

from fastapi import APIRouter, Depends

router = APIRouter(
    dependencies=[Depends([1])]
)

Drag options to blanks, or click blank then click option'

Aapp

Bcommon_dependency

CFastAPI

DRequest

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the app instance instead of the dependency function

Passing a class like FastAPI instead of a function

The dependency function common_dependency must be passed to Depends in the router.

4fill in blank

hard

Fill both blanks to create a route that uses the shared dependency and returns its value.

FastAPI

@router.get("/shared")
async def read_shared(value: str = [1]([2])):
    return {"value": value}

Drag options to blanks, or click blank then click option'

ADepends

Bcommon_dependency

CRequest

DFastAPI

Attempts:

3 left

💡 Hint

Common Mistakes

Using Request or FastAPI instead of Depends

Not passing the dependency function to Depends

The route parameter uses Depends with the common_dependency function to get the shared value.

5fill in blank

hard

Fill all three blanks to include the router in the app with a prefix and shared dependencies.

FastAPI

app.include_router(
    router,
    prefix=[1],
    dependencies=[[2]([3])]
)

Drag options to blanks, or click blank then click option'

A"/items"

BDepends

Ccommon_dependency

D"/users"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong prefix string

Not wrapping the dependency function with Depends

The router is included with prefix '/users' and the shared dependency declared with Depends(common_dependency).

Practice

1. What is the main benefit of using shared dependencies across routers in FastAPI?

easy

A. To disable dependency injection in routers

B. To make the API slower by adding extra layers

C. To force all routers to use the same path prefix

D. To avoid repeating the same dependency code in multiple routers

Solution

1. Step 1: Understand what shared dependencies do

   Shared dependencies allow you to write a dependency once and apply it to many routers, avoiding repetition.

2. Step 2: Identify the benefit in code maintenance

   This reduces duplicate code and makes the project easier to maintain and update.

3. Final Answer:

   To avoid repeating the same dependency code in multiple routers -> Option D

4. Quick Check:

   Shared dependencies = avoid repetition [OK]

Hint: Shared dependencies reduce repeated code across routers [OK]

Common Mistakes:

• Thinking shared dependencies slow down the API
• Confusing shared dependencies with path prefixes
• Believing shared dependencies disable injection

2. Which is the correct way to apply a shared dependency to an APIRouter in FastAPI?

easy

A. router = APIRouter(shared_dependencies=[Depends(common_dep)])

B. router = APIRouter(dependencies=[Depends(common_dep)])

C. router = APIRouter(dependency=Depends(common_dep))

D. router = APIRouter(shared=Depends(common_dep))

Solution

1. Step 1: Recall the APIRouter parameter for dependencies

   The correct parameter name is dependencies, which takes a list of Depends objects.

2. Step 2: Match the syntax to the correct option

   router = APIRouter(dependencies=[Depends(common_dep)]) uses dependencies=[Depends(common_dep)], which is the correct syntax.

3. Final Answer:

   router = APIRouter(dependencies=[Depends(common_dep)]) -> Option B

4. Quick Check:

   dependencies param = correct syntax [OK]

Hint: Use 'dependencies' param with a list of Depends [OK]

Common Mistakes:

• Using 'shared_dependencies' instead of 'dependencies'
• Passing Depends directly without a list
• Using wrong parameter names like 'dependency' or 'shared'

3. Given this code snippet, what will be printed when accessing /items/42?

from fastapi import FastAPI, APIRouter, Depends

app = FastAPI()

def common_dep():
    print("Common dependency called")

router = APIRouter(dependencies=[Depends(common_dep)])

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

app.include_router(router)

medium

A. "Common dependency called" is printed once per request

B. An error occurs because common_dep is not awaited

C. Nothing is printed

D. "Common dependency called" is printed twice per request

Solution

1. Step 1: Understand how dependencies run in FastAPI

   Dependencies listed in the router's dependencies run for every matching request automatically.

2. Step 2: Analyze the print statement in common_dep

   Each time the endpoint is called, common_dep runs and prints "Common dependency called" once.

3. Final Answer:

   "Common dependency called" is printed once per request -> Option A

4. Quick Check:

   Router dependencies run once per request [OK]

Hint: Router dependencies run once per matching request [OK]

Common Mistakes:

• Thinking dependencies run multiple times per request
• Expecting no output from print statements
• Confusing async with sync dependencies causing errors

4. Identify the error in this FastAPI router setup with shared dependencies:

from fastapi import APIRouter, Depends

def verify_token():
    pass

router = APIRouter(dependencies=Depends(verify_token))

@router.get("/secure")
async def secure_endpoint():
    return {"message": "Secure"}

medium

A. Depends should be inside a list in dependencies parameter

B. verify_token must return a value

C. APIRouter cannot have dependencies

D. The route path must start with a slash

Solution

1. Step 1: Check the dependencies parameter type

   The dependencies parameter expects a list of Depends objects, not a single Depends instance.

2. Step 2: Identify the correct syntax

   It should be dependencies=[Depends(verify_token)] to be valid.

3. Final Answer:

   Depends should be inside a list in dependencies parameter -> Option A

4. Quick Check:

   dependencies param requires a list [OK]

Hint: Always wrap Depends in a list for dependencies param [OK]

Common Mistakes:

• Passing Depends directly without list
• Assuming dependencies param accepts single Depends
• Ignoring syntax errors from wrong dependencies format

5. You want to apply a shared dependency that extracts a user from a token to two routers, router_a and router_b. Which approach correctly shares the dependency without repeating code?

hard

A. Define the dependency in each route function separately in both routers

B. Include get_current_user as a parameter in FastAPI() app initialization

C. Add the dependency to each router using the dependencies parameter with Depends(get_current_user)

D. Use global variables to store the user and access it in routes

Solution

1. Step 1: Understand how to share dependencies across routers

   Using the dependencies parameter in APIRouter allows applying a dependency to all routes in that router.

2. Step 2: Apply the dependency to both routers

   Adding dependencies=[Depends(get_current_user)] to both router_a and router_b shares the logic without repeating it in each route.

3. Final Answer:

   Add the dependency to each router using the dependencies parameter with Depends(get_current_user) -> Option C

4. Quick Check:

   Shared dependencies via router dependencies param [OK]

Hint: Use dependencies param on routers to share common logic [OK]

Common Mistakes:

• Repeating dependency code in every route
• Trying to add dependencies in FastAPI app instead of routers
• Using global variables instead of dependency injection