app/utils/emailer.py

import smtplib
from email.mime.text import MIMEText
from app.config import settings
import hashlib
from typing import List, Tuple

def choose_subject(email: str, subject_a: str, subject_b: str) -> Tuple[str, str]:
    # 受信者でハッシュ→安定的にA/B割当（50/50）
    h = hashlib.sha256(email.encode()).digest()[0]
    variant = "A" if (h % 2 == 0) else "B"
    return (subject_a if variant == "A" else subject_b), variant

def send_email(recipients: List[str], subject_a: str, subject_b: str, body_md: str) -> list:
    results = []
    for rcpt in recipients:
        subject, variant = choose_subject(rcpt, subject_a, subject_b)
        msg = MIMEText(body_md, _subtype="plain", _charset="utf-8")
        msg["Subject"] = subject
        msg["From"] = settings.EMAIL_FROM
        msg["To"] = rcpt

        with smtplib.SMTP(settings.SMTP_HOST, settings.SMTP_PORT) as s:
            s.starttls()
            if settings.SMTP_USERNAME and settings.SMTP_PASSWORD:
                s.login(settings.SMTP_USERNAME, settings.SMTP_PASSWORD)
            s.sendmail(settings.SMTP_USERNAME or "", [rcpt], msg.as_string())
        results.append({"to": rcpt, "variant": variant, "subject": subject})
    return results