FastAPI Depdency Injection Example

dep-in.py

from fastapi import FastAPI, Depends
from myservice import UserService, User, UserCreate

app = FastAPI()


class UserService:
  
  def create_user(self):
    return 'doing something that creates user'
  
def user_service() -> UserService:
  return UserService()

@app.post("/user-create-with-dep-inj")
async def create_user(user_service: UserService = Depends(user_service)):
  """
  This is the prefered method as you are injecting something that knows how to create an instance of user service.
  """
    return user_service.create_user()
     
@app.post("/user-create-without-dep-inj")
async def create_user():
  """
  This is a bad implementation as you have a hard depdency on UserService, 
  this makes testing harder and replacement of it harder in the future
  as well this method now requires to know how to create an instance of UserService. If
  user service had other depdencies it needed you would need to use those in this route as well.
  """
  user_service = UserService()
  user_service.create_user()