Why Shared dependencies across routers in FastAPI? - Purpose & Use Cases

The Big Idea

Discover how to stop repeating yourself and make your FastAPI routes smarter with shared dependencies!

The Scenario

Imagine building a web app with many routes, each needing the same setup like checking user login or connecting to a database.

You add this setup code inside every route manually.

The Problem

Writing the same setup code in every route is tiring and easy to forget.

If you want to change the setup, you must update every route, which wastes time and causes bugs.

The Solution

FastAPI lets you define shared dependencies once and apply them to many routes automatically.

This keeps your code clean, consistent, and easy to update.

Before vs After

✗ Before

def route1():
    user = get_current_user()
    db = get_db()
    # route logic

def route2():
    user = get_current_user()
    db = get_db()
    # route logic

✓ After

from fastapi import APIRouter, Depends

shared_deps = [Depends(get_current_user), Depends(get_db)]
router = APIRouter(dependencies=shared_deps)

@router.get('/route1')
async def route1():
    # route logic

@router.get('/route2')
async def route2():
    # route logic

What It Enables

You can manage common setup in one place and have all routes use it automatically, making your app easier to build and maintain.

Real Life Example

A social media app where every page needs to check if the user is logged in and connect to the database before showing content.

Key Takeaways

Writing shared setup code once saves time and avoids mistakes.

FastAPI's shared dependencies keep routes clean and consistent.

Updating shared logic is easy and affects all routes automatically.