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.