$ pipenv install --three
$ pipenv shell

$ pipenv install flask celery

.
├── Pipfile                    # manage our environment
├── Pipfile.lock
├── README.md
├── __init__.py
├── app.py                     # main Flask application implementation
├── config.py                  # to host the configuration
├── requirements.txt           # store our requirements
└── templates
    └── index.html             # the landing page

1 directory, 8 files

from flask import Flask, flash, render_template, request, redirect, url_for

app = Flask(__name__)
app.config.from_object("config")
app.secret_key = app.config['SECRET_KEY']

@app.route('/', methods=['GET', 'POST'])
def index():
    if request.method == 'GET':
        return render_template('index.html')

    elif request.method == 'POST':
        email = request.form['email']
        first_name = request.form['first_name']
        last_name = request.form['last_name']
        message = request.form['message']
        duration = request.form['duration']
        duration_unit = request.form['duration_unit']

        flash(“Message scheduled")
        return redirect(url_for('index'))


if __name__ == '__main__':
    app.run(debug=True)

app.config.from_object("config")

SECRET_KEY = 'very_very_secure_and_secret'
# more config

{% for message in get_flashed_messages() %}
  
{{ message }}
{% endfor %}


    First Name: 
    Last Name: 
    Email: 
    Message: 
    Duration: 

   
      Choose the duration
      Minutes
      Hours
      Days
   

   Submit 

$ pipenv install flask-mail

from flask_mail import Mail, Message

app = Flask(__name__)
app.config.from_object("config")
app.secret_key = app.config['SECRET_KEY']

# set up Flask-Mail Integration
mail = Mail(app)

def send_mail(data):
    """ Function to send emails.
    """
    with app.app_context():
        msg = Message("Ping!",
                    sender="admin.ping",
                    recipients=[data['email']])
        msg.body = data['message']
        mail.send(msg)

# Flask-Mail
MAIL_SERVER = 'smtp.googlemail.com'
MAIL_PORT = 587
MAIL_USE_TLS = True
MAIL_USERNAME = 'mail-username'
MAIL_PASSWORD = 'mail-password'

# Existing imports are maintained
from celery import Celery

# Flask app and flask-mail configuration truncated

# Set up celery client
client = Celery(app.name, broker=app.config['CELERY_BROKER_URL'])
client.conf.update(app.config)

# Add this decorator to our send_mail function
@client.task
def send_mail(data):
    # Function remains the same

@app.route('/', methods=['GET', 'POST'])
def index():
    if request.method == 'GET':
        return render_template('index.html')

    elif request.method == 'POST':
        data = {}
        data['email'] = request.form['email']
        data['first_name'] = request.form['first_name']
        data['last_name'] = request.form['last_name']
        data['message'] = request.form['message']
        duration = int(request.form['duration'])
        duration_unit = request.form['duration_unit']

        if duration_unit == 'minutes':
            duration *= 60
        elif duration_unit == 'hours':
            duration *= 3600
        elif duration_unit == 'days':
            duration *= 86400

        send_mail.apply_async(args=[data], countdown=duration)
        flash(f"Email will be sent to {data['email']} in {request.form['duration']} {duration_unit}")

        return redirect(url_for('index'))

CELERY_BROKER_URL = 'redis://localhost:6379/0'
CELERY_RESULT_BACKEND = 'redis://localhost:6379/0'

$ python app.py

# start the virtualenv
$ pipenv shell
$ celery worker -A app.client --loglevel=info

[2019-10-23 16:27:25,399: INFO/MainProcess] Received task: app.send_mail[d65025c8-a291-40d0-aea2-e816cb40cd78]  ETA:[2019-10-23 13:29:25.170622+00:00]

$ pipenv install flower

$ pipenv shell
$ flower -A app.client --port=5555