app.py

from flask import Flask, redirect, url_for, session, request, render_template
from dropbox.client import DropboxClient, DropboxOAuth2Flow
import dropbox
import redis
from datetime import datetime, timedelta
import os

# Token for the account holding the images
my_token = os.environ['TOKEN']

redis_url = os.environ['REDISTOGO_URL']
redis_client = redis.from_url(redis_url)

# App key and secret from the App console (dropbox.com/developers/apps)
APP_KEY = os.environ['APP_KEY']
APP_SECRET = os.environ['APP_SECRET']

app = Flask(__name__)
app.debug = True

# A random secret used by Flask to encrypt session data cookies
app.secret_key = os.environ['FLASK_SECRET_KEY']

def get_callback_url():
    '''Generate a proper callback URL, forcing HTTPS if not running locally'''
    url = url_for(
        'callback',
        _external=True,
        _scheme='http' if request.host.startswith('127.0.0.1') else 'https'
    )
    return url

def get_flow():
    return DropboxOAuth2Flow(
        APP_KEY,
        APP_SECRET,
        get_callback_url(),
        session,
        'dropbox-csrf-token')

def get_answer_url():
    '''Based on the current date, generate a media link for the current photo (<date>.jpg).'''
    pacific_now = datetime.utcnow() + timedelta(hours=-8)
    return DropboxClient(my_token).media(pacific_now.strftime('%Y-%m-%d.jpg'))['url']

@app.route('/')
def index():
    # Pass a URL for today's image into the view.
    return render_template('index.html', url=get_answer_url())

@app.route('/login')
def login():
    # A bit of JavaScript on the client passes a 'tz' parameter specifying the
    # user's time zone. Pass that through the OAuth flow in the state parameter.
    tz = request.args.get('tz', '0')
    return redirect(get_flow().start(tz))

cache = {}
def get_copy_ref(token, date):
    '''For today's photo, return a copy_ref that will be used to copy the file
       into users' accounts. Cache these for efficiency when processing a lot of
       accounts on the same day (which happens in the cron job).'''
    if date not in cache:
        cache[date] = DropboxClient(token).create_copy_ref(date + '.jpg')['copy_ref']
    return cache[date]

def update_if_needed(uid, r, my_token):
    '''For a given user, copy a new photo into their Dropbox if needed.'''

    # Get the user's access token from Redis.
    access_token = redis_client.hget('tokens', uid)

    # Get the time zone, or use Pacific time by default.
    tz = redis_client.hget('timezones', uid) or '-8'

    date = (datetime.utcnow() + timedelta(hours=int(tz))).strftime('%Y-%m-%d')

    # If the user hasn't been updated yet today, copy in a new photo.
    if redis_client.hget('last_update', uid) != date:

        # Get a copy ref from the master account
        copy_ref = get_copy_ref(my_token, date)

        client = DropboxClient(access_token)
        try:
            # Add the photo
            client.add_copy_ref(copy_ref, 'Is %s Christmas.jpg' % date)
        except:
            # Ignore all errors! Probably a bad idea, but the most common
            # error is that there's a conflict because the user actually
            # already has this file. TODO: Catch that specifically. :-)
            pass

        # Track the last update so we don't revisit this user until tomorrow.
        redis_client.hset('last_update', uid, date)

@app.route('/callback')
def callback():
    '''Callback function for when the user returns from OAuth.'''

    # Extract and store the access token, user ID, and time zone.
    access_token, uid, tz = get_flow().finish(request.args)
    redis_client.hset('tokens', uid, access_token)
    redis_client.hset('timezones', uid, tz)

    # For new users, this should give them today's photo.
    update_if_needed(uid, r, my_token)

    return render_template('done.html', url=get_answer_url())

@app.route('/cron')
def cron():
    '''Cron job, triggered remotely on a regular basis by hitting this URL.'''

    # Update any users that need it.
    for uid in redis_client.hkeys('tokens'):
        update_if_needed(uid, r, my_token)

    return 'Okay.'

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

done.html

<!-- To be placed in templates/done.html -->
{% extends 'layout.html' %}

{% block body %}
<p>That's it! You should now see an image in the folder "<code>Dropbox/Apps/Is it Christmas</code>" each day.</p>
{% endblock %}

index.html

<!-- To be placed in templates/index.html -->
{% extends 'layout.html' %}
{% block body %}
    <a class="button" href="login">Connect to Dropbox for daily updates!</a>
    <script>
        var links = document.getElementsByTagName('a');
        var tz = -new Date().getTimezoneOffset()/60;
        for (var i = 0; i < links.length; i++) {
            links[i].href += '?tz=' + tz;
        }
    </script>
{% endblock %}

layout.html

<!-- To be placed in templates/layout.html -->
<!doctype html>
<html>
<head>
    <title>Is it Christmas?</title>
    <link rel="stylesheet" href="/static/style.css">
</head>
<body>
    <h1>Is it Christmas?</h1>
    <a class="image" href="/"><img src="{{url}}"></a>
    {% block body %}{% endblock %}
</body>
</html>

style.css

/* To be placed in static/style.css */
body { font-family: Helvetica, 'Helvetica Neue', 'Segoe UI', Arial; }
h1 { text-align: center; margin: 50px; padding-top: 0; font-weight: normal; }
code { font-family: Monaco, Menlo, Consolas, monospace; }
a.image {
    width: 612px;
    display: block;
    margin: 0 auto;
}
a.image img { border: none; outline: none; }
a.button {
    text-decoration: none;
    text-align: center;
    width: 300px;
    outline: none;
    color: white;
    display: block;
    margin: 50px auto;
    background-color: #007ee5;
    border: none;
    font-size: 16px;
    line-height: 24px;
    padding: 14px;
    -webkit-box-shadow: inset 0px -3px 1px rgba(0, 0, 0, 0.45);
    -moz-box-shadow: inset 0px -3px 1px rgba(0, 0, 0, 0.45);
    box-shadow: inset 0px -3px 1px rgba(0, 0, 0, 0.45);
    -webkit-border-radius: 3px;
    -moz-border-radius: 3px;
    border-radius: 3px;
}
a.button:active {
    position: relative; top: 3px;
    -webkit-box-shadow: inset 0px -3px 1px rgba(255, 255, 255, 1), inset 0 0px 3px rgba(0, 0, 0, 0.9);
    -moz-box-shadow: inset 0px -3px 1px rgba(255, 255, 255, 1), inset 0 0px 3px rgba(0, 0, 0, 0.9);
    box-shadow: inset 0px -3px 1px rgba(255, 255, 255, 1), inset 0 0px 3px rgba(0, 0, 0, 0.9);
}
a.button:active:after { content: ""; width: 100%; height: 3px; background: #fff; position: absolute; bottom: -1px; left: 0; }
p { text-align: center; font-size: 18px; margin-top: 50px; }