# main.py
import os
from contextlib import asynccontextmanager
from fastapi import FastAPI, HTTPException
from pydantic import BaseModel
from maildeno import AsyncMaildenoClient, MaildenoError

@asynccontextmanager
async def lifespan(app: FastAPI):
    app.state.maildeno = AsyncMaildenoClient(
        api_key=os.environ["MAILDENO_API_KEY"]
    )
    yield
    await app.state.maildeno.aclose()

app = FastAPI(lifespan=lifespan)

class RenderRequest(BaseModel):
    template_id: str
    name: str
    plan: str

@app.post("/api/render-email")
async def render_email(req: RenderRequest, request: Request):
    try:
        html = await request.app.state.maildeno.render_html(
            req.template_id,
            {
                "merge_tags": {"text": {"name": req.name}},
                "context":    {"plan": req.plan},
            },
        )
        return {"html": html}
    except MaildenoError as err:
        raise HTTPException(
            status_code=err.status or 500,
            detail={"error": err.code, "message": err.message},
        )

# app.py
import os
from flask import Flask, jsonify, request
from maildeno import MaildenoClient, MaildenoError

app = Flask(__name__)
maildeno = MaildenoClient(api_key=os.environ["MAILDENO_API_KEY"])

@app.post("/api/render-email")
def render_email():
    data = request.get_json()
    try:
        html = maildeno.render_html(
            data["template_id"],
            {
                "merge_tags": {"text": {"name": data["name"]}},
                "context":    {"plan": data["plan"]},
            },
        )
        return jsonify(html=html)
    except MaildenoError as err:
        return jsonify(error=err.code, message=err.message), err.status or 500

# views.py
import json
import os
from django.http import JsonResponse
from django.views.decorators.http import require_POST
from maildeno import MaildenoClient, MaildenoError

maildeno = MaildenoClient(api_key=os.environ["MAILDENO_API_KEY"])

@require_POST
def render_email(request):
    data = json.loads(request.body)
    try:
        html = maildeno.render_html(
            data["template_id"],
            {"merge_tags": {"text": {"name": data["name"]}}},
        )
        return JsonResponse({"html": html})
    except MaildenoError as err:
        return JsonResponse(
            {"error": err.code, "message": err.message},
            status=err.status or 500,
        )

# tasks.py
import os
from celery import Celery
from maildeno import MaildenoClient, MaildenoError

app = Celery("tasks")
maildeno = MaildenoClient(api_key=os.environ["MAILDENO_API_KEY"])

@app.task(bind=True, max_retries=3)
def send_welcome_email(self, user_id: int, name: str, email: str):
    try:
        html = maildeno.render_html(
            "welcome-template-id",
            {"merge_tags": {"text": {"name": name}}},
        )
        # send html via your email provider ...
    except MaildenoError as err:
        if err.code in ("NETWORK_ERROR", "TIMEOUT"):
            raise self.retry(exc=err, countdown=2 ** self.request.retries)
        raise

# backfill_emails.py
import os
from maildeno import MaildenoClient

with MaildenoClient(api_key=os.environ["MAILDENO_API_KEY"]) as client:
    for user in get_all_users():
        html = client.render_html(
            "onboarding-template",
            {"merge_tags": {"text": {"name": user.name}}},
        )
        send_email(user.email, html)