1. What is a Dependency?#

A dependency is just a function. FastAPI will run this function before your endpoint runs and pass the result into your route.

2. Implementation: Injecting the Mock DB#

Instead of accessing db globally, we can create a dependency that “provides” the database to our routes.

from fastapi import FastAPI, Depends
from typing import Annotated

app = FastAPI()

# 1. Define the dependency
def get_db():
    # In a real app, this is where you'd open a DB session
    return db

# 2. Use it in a route
@app.get("/items/")
async def read_items(database: Annotated[list, Depends(get_db)]):
    return {"data": database}

3. Why use Annotated?#

You’ll notice I used Annotated. This is the modern, recommended way to write FastAPI dependencies. It makes the code more readable and plays nicely with other Python tools (like linters and static analyzers).

4. Benefits I Discovered Today:#

• Dry (Don’t Repeat Yourself): If I want to add logging to every database call, I only have to change it in the get_db function.
• Security: You can use dependencies to check if a user is logged in before the route even begins.
• Validation: Dependencies can have their own Query and Path parameters, meaning they can validate data before it ever reaches your main logic.

🛠️ Implementation Checklist#

• Created a get_db dependency function.
• Refactored the POST and GET routes to use Depends(get_db).
• Verified that the API still functions exactly as before (but with cleaner code!).
• Explored how dependencies show up in Swagger UI (they look like regular parameters).

📚 Resources#

1. Official Docs: FastAPI Dependencies - First Steps
2. Book: FastAPI: Modern Python Web Development (Chapter 5: Dependency Injection).