Shared dependencies across routers in FastAPI

Introduction

Shared dependencies let you reuse code for things like authentication or database access across different parts of your app. This keeps your code clean and easy to manage.

When multiple API routes need the same authentication check.

When several routers require access to the same database session.

When you want to apply common headers or parameters to many routes.

When you want to keep your code DRY (Don't Repeat Yourself) by sharing logic.

When organizing a large app with many routers that share setup steps.

Syntax

FastAPI

from fastapi import FastAPI, APIRouter, Depends

# Define a dependency function
def common_dependency():
    return "Shared value"

# Create routers
router1 = APIRouter(dependencies=[Depends(common_dependency)])
router2 = APIRouter(dependencies=[Depends(common_dependency)])

# Use routers in app
app = FastAPI()
app.include_router(router1)
app.include_router(router2)

Use dependencies=[Depends(your_dependency)] when creating an APIRouter to share dependencies.

Dependencies listed here run before any route in that router.

Examples

This router requires token verification for all its routes.

FastAPI

from fastapi import APIRouter, Depends

def verify_token():
    # Imagine token verification here
    return True

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

All routes in this router get a database session automatically.

FastAPI

from fastapi import APIRouter, Depends

def get_db():
    db = "db session"
    try:
        yield db
    finally:
        pass  # close db

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

Sample Program

This example shows two routers sharing the same dependency that could add a common header or perform a check. Both routers have routes that return simple messages.

FastAPI

from fastapi import FastAPI, APIRouter, Depends
from fastapi.responses import JSONResponse

# Dependency function
async def common_header():
    return {"X-Common-Header": "Value"}

# Router 1 with shared dependency
router1 = APIRouter(dependencies=[Depends(common_header)])

@router1.get("/items")
async def read_items():
    return {"message": "Items from router1"}

# Router 2 with same shared dependency
router2 = APIRouter(dependencies=[Depends(common_header)])

@router2.get("/users")
async def read_users():
    return {"message": "Users from router2"}

# Create app and include routers
app = FastAPI()
app.include_router(router1, prefix="/r1")
app.include_router(router2, prefix="/r2")

Important Notes

Shared dependencies run for every request to routes in that router.

You can also add dependencies globally to the FastAPI app for all routes.

Dependencies can return values or perform actions like authentication or logging.

Summary

Shared dependencies help reuse code across multiple routers.

They keep your app organized and avoid repeating code.

Use dependencies=[Depends(your_dependency)] when creating routers.

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