python 49 lines · 8 steps

Sending welcome emails off the request thread in Flask

A signup route creates a user and dispatches a welcome email on a background thread so the response returns immediately.

Explained by highlit
Flask Intermediate background-threads app-context email non-blocking blueprints
1import smtplib
2from email.message import EmailMessage
3from threading import Thread
4 
5from flask import Blueprint, current_app, jsonify, request
6 
7users = Blueprint("users", __name__)
8 
9 
10def _send_email(app, recipient, name):
11 with app.app_context():
12 msg = EmailMessage()
13 msg["Subject"] = "Welcome aboard!"
14 msg["From"] = current_app.config["MAIL_FROM"]
15 msg["To"] = recipient
16 msg.set_content(
17 f"Hi {name},\n\nThanks for signing up. We're glad to have you."
18 )
19 try:
20 with smtplib.SMTP(
21 current_app.config["SMTP_HOST"],
22 current_app.config["SMTP_PORT"],
23 timeout=10,
24 ) as server:
25 server.starttls()
26 server.login(
27 current_app.config["SMTP_USER"],
28 current_app.config["SMTP_PASSWORD"],
29 )
30 server.send_message(msg)
31 except Exception:
32 current_app.logger.exception("Failed to send welcome email to %s", recipient)
33 
34 
35def send_welcome_email_async(recipient, name):
36 app = current_app._get_current_object()
37 Thread(target=_send_email, args=(app, recipient, name), daemon=True).start()
38 
39 
40@users.route("/signup", methods=["POST"])
41def signup():
42 data = request.get_json(force=True)
43 email = data["email"]
44 name = data.get("name", "there")
45 
46 user = User.create(email=email, name=name)
47 send_welcome_email_async(user.email, user.name)
48 
49 return jsonify(id=user.id, email=user.email), 201
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Offloading slow I/O like SMTP to a background thread keeps HTTP responses fast.
  2. 2Flask's request-bound context doesn't exist in new threads, so you must push an app context manually.
  3. 3Capture the real app object before spawning a thread, since context-local proxies won't resolve outside the request.

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