Add authentication to dash apps

"""
This submodule is inspired from the dash-flask-login library at:

https://github.com/gaw89/dash-flask-login

It is unlicensed.

To implement this in your dash app, add this file auth.py to your superproject
and do something like this in your server script:

    from superproject.auth import FlaskLoginAuth
    server = Flask("superproject")
    app    = Dash(name='superproject', server=server, url_base_pathname='/')
    users  = [('steven', 'vc98zq23jkhjyxcvoi')]
    auth   = FlaskLoginAuth(app, users=users)

Be sure to create /assets/login.html with a log-in form for it to work.
"""

# Modules #
import os, hashlib, flask

# Flask #
from flask import request, redirect, url_for
from flask_login import login_required, LoginManager, UserMixin, login_user

###############################################################################
class DefaultUser(UserMixin):

    def __init__(self, name, password, hash_function):
        self.id       = name.lower()
        self.username = name.lower()
        self.password = hash_function(password)

    def __eq__(self, other):
        return self.id == other.id

###############################################################################
class UserMap(object):

    def __init__(self, users):
        """*users* is a list of DefaultUser objects"""
        self.users = users
        self.user_map = {}
        for u in self.users: self.user_map.update({u.id: u})

    def get_user(self, id):
        try:    return self.user_map[id]
        except: return None

###############################################################################
class FlaskLoginAuth():
    def __init__(self, app, users):
        """
        app: A Dash object to be login-protected
        users: Should be a list of tuples of the format (<USERNAME>, <PASSWORD>)
               where each element is a unicode string.
        This will be used to create a list of DefaultUser objects.
        """
        # Basic attributes #
        self.server = app.server

        # Users #
        self.users = UserMap([DefaultUser(u[0], u[1], self.hash_function) for u in users])

        # Alter the view functions of the server to require login #
        self.server.view_functions['/view'] = login_required(self.server.view_functions['/view'])

        # Setup the LoginManager for the server #
        self.login_manager = LoginManager()
        self.login_manager.init_app(self.server)
        self.login_manager.login_view = "/login"

        # Secret Key #
        self.server.config.update(SECRET_KEY = os.urandom(12))

        # Callback to reload the user object #
        @self.login_manager.user_loader
        def load_user(userid):
            return self.users.get_user(userid)

        # The default templates #
        current_directory = os.path.dirname(os.path.abspath(__file__))
        self.login_html = current_directory + '/assets/login.html'

        # Rule #
        self.server.add_url_rule(
            '/login',
            view_func = self.default_login_view,
            methods = ['GET', 'POST']
        )

        # Base route goes to login #
        @self.server.route('/', methods=['GET'])
        def save():
            return redirect(url_for('default_login_view'))

    def hash_function(self, string):
        return hashlib.md5().update(string.encode('utf-8')).hexdigest()

    def default_login_view(self):
        if request.method == 'POST':
            username = request.form['username'].lower()
            password = request.form['password']
            password = self.hash_function(password)
            user     = self.users.get_user(username)
            if user:
                if password == user.password:
                    login_user(user)
                    return redirect(flask.url_for('/view'))
        return flask.send_file(self.login_html)