SendGrid with FastAPI: BackgroundTasks so mail does not block the response

import os, sendgrid
from fastapi import FastAPI, BackgroundTasks
from sendgrid.helpers.mail import Mail

app = FastAPI()
sg = sendgrid.SendGridAPIClient(api_key value os.environ.get('SENDGRID_API_KEY'))

def send_welcome(to_email: str):
  message = Mail(
    from_email='[YOUR_VERIFIED_SENDER]',  # verified sender
    to_emails=to_email,
    subject='Welcome',
    plain_text_content='Welcome aboard')
  response = sg.send(message)
  if response.status_code != 202:
    print('sendgrid failed', response.status_code, response.body)

@app.post('/signup')
def signup(email: str, background_tasks: BackgroundTasks):
  background_tasks.add_task(send_welcome, email)
  return {'ok': True}

Notes:
1. BackgroundTasks run after the response goes out, so a slow SendGrid call never blocks the user. But they die with the worker: for anything you cannot afford to lose, use a real queue.
2. Create the client once at startup, not per request.
3. Validate the email address in the endpoint; a 400 from SendGrid on a bad address is avoidable.
4. For tests, call the send function with sandbox mode enabled and assert 200.