dotenv

@@ -16,3 +16,19 @@
 
 FLASK_APP=tjts5901.app
 FLASK_DEBUG=1
+
+# Enable rich logging for more human readable log output. Requires installing
+# `rich` and `flask-rich` packages.
+#RICH_LOGGING=1
+
+# Mongodb connection string
+MONGO_URL=mongodb://mongodb:27017/tjts5901
+
+# Sentry logging. Fetch the DSN ingest key: <https://docs.sentry.io/platforms/python/guides/flask/>
+#SENTRY_DSN=https://ABC@XYZ.ingest.sentry.io/123
+# Setup sentry environment to separate development issues from production. This variable name is
+# maybe set by gitlab pipeline. <https://docs.gitlab.com/ee/ci/environments/>
+CI_ENVIRONMENT_NAME=development
+
+# Setup CI environment url to point on localhost for testing purposes.
+CI_ENVIRONMENT_URL=http://localhost:5001

pyproject.toml

@@ -20,8 +20,6 @@ authors = [
   { name="University of Jyväskylä" },
   # { name="Team Member", email="team.member@student.jyu.fi" }
 ]
-
-# Version is detected from git, and dependencies from older style "requirements.txt" file.
 dynamic = ["dependencies"]
 
 [tool.setuptools.dynamic]
@@ -30,21 +28,35 @@ dependencies = {file = ["requirements.txt"]}
 [project.optional-dependencies]
 test = [
   "pytest",
+  "coverage",
+  "coverage[toml]",
+  "requests",
+  "faker",
 ]
 docs = [
   "mkdocs",
   "mkdocs-material",
   "mkdocs-git-revision-date-localized-plugin",
+  "mkdocs-macros-plugin",
 ]
 
 [build-system]
 requires = ["setuptools>=61.0"]
 build-backend = "setuptools.build_meta"
 
-[tool.setuptools_scm]
-# Empty segment required to activate scm
-
 [tool.pytest.ini_options]
 testpaths = [
     "tests",
 ]
+filterwarnings = [
+  ## Silence json encoder warnings for Flask >=2.3
+  # "ignore::DeprecationWarning:mongoengine.connection",
+  # "ignore::DeprecationWarning:flask_mongoengine.json",
+  # "ignore::DeprecationWarning:flask.json.provider",
+]
+
+[tool.coverage.run]
+branch = true
+source_pkgs = [
+    "tjts5901",
+]

requirements.txt

+# Support library for versioning
+importlib-metadata
+
 # Framework and libraries
-flask
+flask==2.2.2
 python-dotenv
+flask-login
+flask-babel
+
+flask-mongoengine==1.0
+
+CurrencyConverter
+
+Flask-APScheduler
 
 # Git hooks
 pre-commit
 
+# Sentry for error reporting
+sentry-sdk[flask]
+sentry-sdk[pymongo]
+
 # More production-ready web server
-#gunicorn
+gunicorn

src/tjts5901/__init__.py

@@ -6,7 +6,14 @@ JYU TJTS5901 Course project
 from importlib_metadata import (PackageNotFoundError,
                                 version)
 
+from .app import create_app
+
 try:
     __version__ = version(__name__)
 except PackageNotFoundError:
     __version__ = "unknown"
+
+__all__ = [
+    "create_app",
+    "__version__",
+]

src/tjts5901/app.py

-from importlib.metadata import version
+"""
+Flask Application
+=================
+
+This is the default entrypoint for our application.
+Flask tutorial: https://flask.palletsprojects.com/en/2.2.x/tutorial/
+
+"""
+
+import logging
 from os import environ
+import os
+from typing import Dict, Literal, Optional
 
 from dotenv import load_dotenv
 from flask import (
@@ -9,34 +20,78 @@ from flask import (
     request,
 )
 
+from flask_babel import _
+
+from .utils import get_version
+from .db import init_db
+from .i18n import init_babel
+
+logger = logging.getLogger(__name__)
 
-def create_app(name=__name__) -> Flask:
+
+def create_app(config: Optional[Dict] = None) -> Flask:
     """
     Application factory for creating a new Flask instance.
 
     :param name: The name of the application.
     """
-    new_app = Flask(name)
+    flask_app = Flask(__name__, instance_relative_config=True)
 
-    return new_app
+    flask_app.config.from_mapping(
+        SECRET_KEY='dev',
+        BRAND=_("Hill Valley DMC dealership"),
+    )
 
+    # load the instance config, if it exists, when not testing
+    if config is None:
+        flask_app.config.from_pyfile('config.py', silent=True)
+    else:
+        flask_app.config.from_mapping(config)
 
-def get_version() -> str:
-    """
-    Get the version of the application.
+    # Initialize logging early, so that we can log the rest of the initialization.
+    from .logging import init_logging  # pylint: disable=import-outside-toplevel
+    init_logging(flask_app)
 
-    This is useful for checking the version of the application, and for
-    monitoring the application.
-    """
+    # ensure the instance folder exists
+    try:
+        os.makedirs(flask_app.instance_path)
+    except OSError:
+        pass
+
+    # Initialize the Flask-Babel extension.
+    init_babel(flask_app)
+
+    # Initialize the database connection.
+    init_db(flask_app)
 
-    # Get the version from the environment varible. It's setup by CI/CD pipeline.
-    ver = environ.get("CI_COMMIT_SHA", None)
+    # Initialize the scheduler.
+    from .scheduler import init_scheduler  # pylint: disable=import-outside-toplevel
+    init_scheduler(flask_app)
 
-    if ver is None:
-        # Get the version from the package.
-        ver = version(__name__)
+    @flask_app.route('/debug-sentry')
+    def trigger_error():
+        division_by_zero = 1 / 0
 
-    return str(ver)
+    # a simple page that says hello
+    @flask_app.route('/hello')
+    def hello():
+        return _('Hello, World!')
+
+    from .auth import init_auth
+    init_auth(flask_app)
+
+    from .notification import init_notification
+    init_notification(flask_app)
+
+    from .currency import init_currency
+    init_currency(flask_app)
+
+    from . import items
+    flask_app.register_blueprint(items.bp)
+    flask_app.register_blueprint(items.api)
+    flask_app.add_url_rule('/', endpoint='index')
+
+    return flask_app
 
 
 # Load environment variables from .env file, if present. See the `dotenv` file for a
@@ -44,16 +99,10 @@ def get_version() -> str:
 load_dotenv()
 
 # Create the Flask application.
-app = create_app()
+flask_app = create_app()
 
-
-@app.route("/")
-def index():
-    return "Hello, world!"
-
-
-@app.route("/server-status")
-def server_status() -> Response:
+@flask_app.route("/server-info")
+def server_info() -> Response:
     """
     A simple endpoint for checking the status of the server.
 
@@ -61,8 +110,31 @@ def server_status() -> Response:
     running correctly.
     """
 
+    # Test for database connection
+    database_ping = False
+    try:
+        from .db import db  # pylint: disable=import-outside-toplevel
+        database_ping = db.connection.admin.command('ping').get("ok", False) and True
+    except Exception as exc:  # pylint: disable=broad-except
+        logger.warning("Error querying mongodb server: %r", exc,
+                       exc_info=True,
+                       extra=flask_app.config.get_namespace("MONGODB"))
+
+    # Check for sentry
+    sentry_available = False
+    try:
+        from sentry_sdk import Hub
+        sentry_available = True if Hub.current.client else False
+    except ImportError:
+        logger.warning("Sentry package is not installed")
+    except TypeError:
+        logger.info("Sentry is not integrated")
+
     response = {
+        "database_connectable": database_ping,
+        'sentry_available': sentry_available,
         "version": get_version(),
+        "build_date": environ.get("BUILD_DATE", None)
     }
 
     # Response with pong if ping is provided.

src/tjts5901/auth.py

+from datetime import datetime
+import functools
+import logging
+
+from flask import (
+    Blueprint, flash, redirect, render_template, request, session, url_for, abort
+)
+from flask_login import (
+    LoginManager,
+    login_user,
+    logout_user,
+    login_required,
+    current_user,
+)
+from flask_babel import _
+from babel.dates import get_timezone
+from werkzeug.security import check_password_hash, generate_password_hash
+from sentry_sdk import set_user
+
+from .models import AccessToken, Bid, User, Item
+
+from mongoengine import DoesNotExist
+from mongoengine.queryset.visitor import Q
+
+bp = Blueprint('auth', __name__, url_prefix='/auth')
+logger = logging.getLogger(__name__)
+
+
+def init_auth(app):
+    """
+    Integrate authentication into the application.
+    """
+    app.register_blueprint(bp)
+
+    login_manager = LoginManager()
+    login_manager.login_view = 'auth.login'
+    login_manager.user_loader(load_logged_in_user)
+
+    app.config['AUTH_HEADER_NAME'] = 'Authorization'
+    login_manager.request_loader(load_user_from_request)
+
+    login_manager.init_app(app)
+
+    logger.debug("Initialized authentication")
+
+
+def load_user_from_request(request):
+    """
+    Load a user from the request.
+
+    This function is used by Flask-Login to load a user from the request.
+    """
+    api_key = request.headers.get("Authorization")
+
+    if api_key:
+        api_key = api_key.replace("Bearer ", "", 1)
+        try:
+            token = AccessToken.objects.get(token=api_key)
+            if token.expires and token.expires < datetime.utcnow():
+                logger.warning("Token expired: %s", api_key)
+                return None
+            # User is authenticated
+
+            token.last_used_at = datetime.utcnow()
+            token.save()
+            logger.debug("User authenticated via token: %r", token.user.email, extra={
+                "user": token.user.email,
+                "user_id": str(token.user.id),
+                "token": token.token,
+            })
+            return token.user
+        except DoesNotExist:
+            logger.error("Token not found: %s", api_key)
+
+    return None
+
+
+def load_logged_in_user(user_id):
+    """
+    Load a user from the database, given the user's id.
+    """
+    try:
+        user = User.objects.get(id=user_id)
+        set_user({"id": str(user.id), "email": user.email})
+    except DoesNotExist:
+        logger.error("User not found: %s", user_id)
+        return None
+
+    return user
+
+
+def get_user_by_email(email: str) -> User:
+    """
+    Get a user from the database, given the user's email.
+
+    If the email is 'me', then the current user is returned.
+
+    :param email: The email of the user to get.
+    """
+
+    if email is None:
+        abort(404)
+
+    if email == "me" and current_user.is_authenticated:
+        email = current_user.email
+
+    try:
+        user = User.objects.get_or_404(email=email)
+    except DoesNotExist:
+        logger.error("User not found: %s", email)
+        abort(404)
+
+    return user
+
+
+@bp.route('/register', methods=('GET', 'POST'))
+def register():
+    if request.method == 'POST':
+        print("Registering user...")
+        email = request.form['email']
+        password = request.form['password']
+        password2 = request.form['password2']
+        terms = request.form.get('terms', False)
+        error = None
+
+        if not email:
+            error = 'Username is required.'
+        elif not password:
+            error = 'Password is required.'
+        elif password != password2:
+            error = 'Passwords do not match.'
+        elif not terms:
+            error = 'You must agree to the terms.'
+
+        timezone = None
+        try:
+            if user_tz := request.form.get('timezone', None):
+                timezone = get_timezone(user_tz).zone
+
+        except Exception as exc:
+            logger.debug("Error getting timezone from user data: %s", exc)
+            timezone = None
+
+        if error is None:
+            try:
+                user = User(
+                    email=email,
+                    password=generate_password_hash(password),
+                    timezone=timezone,
+                )
+                user.save()
+                flash("You have been registered. Please log in.")
+
+            except Exception as exc:
+                error = f"Error creating user: {exc!s}"
+            else:
+                return redirect(url_for("auth.login"))
+
+        print("Could not register user:", error)
+        flash(error)
+
+    return render_template('auth/register.html')
+
+
+@bp.route('/login', methods=('GET', 'POST'))
+def login():
+    if request.method == 'POST':
+        email = request.form['email']
+        password = request.form['password']
+
+        user = None
+        error = None
+        try:
+            user = User.objects.get(email=email)
+        except DoesNotExist:
+            error = 'Incorrect username.'
+
+        if user is None:
+            error = 'Incorrect username.'
+        elif not check_password_hash(user['password'], password):
+            error = 'Incorrect password.'
+
+        if error is None:
+            remember_me = bool(request.form.get("remember-me", False))
+            if login_user(user, remember=remember_me):
+
+                flash(f"Hello {email}, You have been logged in.")
+
+                next = request.args.get('next')
+                # Better check that the user actually clicked on a relative link
+                # or else they could redirect you to a malicious website!
+                if next is None or not next.startswith('/'):
+                    next = url_for('index')
+
+                return redirect(next)
+            else:
+                error = "Error logging in."
+
+        logger.info("Error logging user in: %r: Error: %s", email, error)
+        flash(error)
+
+    return render_template('auth/login.html')
+
+
+@bp.route('/logout')
+@login_required
+def logout():
+    """
+    Log out the current user.
+
+    Also removes the "remember me" cookie.
+    """
+    logout_user()
+    flash("You have been logged out.")
+    return redirect(url_for('index'))
+
+
+@bp.route('/profile', defaults={'email': 'me'})
+@bp.route('/profile/<email>')
+@login_required
+def profile(email):
+    """
+    Show the user's profile page for the given email.
+
+    If the email is 'me', then the current user's profile is shown.
+    """
+
+    user: User = get_user_by_email(email)
+
+    # List the items user has created
+    items = Item.objects(seller=user).all()
+
+    # List the items user has won
+    # TODO: Could be done smarter with a join
+    bids = Bid.objects(bidder=user).only("id").all()
+    won_items = Item.objects(winning_bid__in=bids).all()
+
+    return render_template('auth/profile.html', user=user, items=items, won_items=won_items)
+
+
+@bp.route('/profile/<email>/token', methods=('GET', 'POST'), defaults={'email': 'me'})
+@login_required
+def user_access_tokens(email):
+    """
+    Show the user's tokens page for the given email.
+    """
+
+    user: User = get_user_by_email(email)
+    # Fetch all the user tokens that are active or have no expire date
+    tokens = AccessToken.objects(Q(expires__gte=datetime.now()) | Q(expires=None), user=user).all()
+
+    token = None
+    if request.method == 'POST':
+        try:
+            name = request.form['name']
+
+            if expires := request.form.get('expires'):
+                expires = datetime.fromisoformat(expires)
+            else:
+                expires = None
+
+            token = AccessToken(
+                user=user,
+                name=name,
+                expires=expires,
+            )
+            token.save()
+        except KeyError as exc:
+            logger.debug("Missing required field: %s", exc)
+            flash(_("Required field missing"))
+        except Exception as exc:
+            logger.exception("Error creating token: %s", exc)
+            flash(_("Error creating token: %s") % exc)
+        else:
+            flash(_("Created token: %s") % token.name)
+
+    return render_template('auth/tokens.html', user=user, tokens=tokens, token=token)
+
+
+@bp.route('/profile/<email>/token/<id>', methods=('POST',))
+def delete_user_access_token(email, id):
+    """
+    Delete an access token.
+    """
+    user = get_user_by_email(email)
+    token = AccessToken.objects.get_or_404(id=id)
+
+    if token.user != user:
+        logger.warning("User %s tried to delete token %s", user.email, token.name, extra={
+            "user": user.email,
+            "token": str(token.id),
+            "token_user": token.user.email,
+        })
+        abort(403)
+
+    token.delete()
+
+    flash(f"Deleted token {token.name}")
+    return redirect(url_for('auth.user_access_tokens', email=token.user.email))

src/tjts5901/currency.py

+"""
+Currency module.
+
+This module contains the currency module, which is used to convert currencies.
+
+Uses the ECB (European Central Bank) as the source of currency conversion rates.
+
+To update the currency conversion rates, run the following command:
+    $ flask update-currency-rates
+
+"""
+
+from decimal import Decimal
+import logging
+import os
+from pathlib import Path
+from zipfile import ZipFile
+import urllib.request
+import click
+from currency_converter import (
+    SINGLE_DAY_ECB_URL,
+    CurrencyConverter,
+)
+
+from flask_babel import (
+    get_locale,
+    format_currency,
+)
+
+from babel.numbers import get_territory_currencies, parse_decimal
+
+from flask import (
+    Flask,
+    current_app,
+    render_template,
+)
+
+from markupsafe import Markup
+
+from .auth import current_user
+
+
+REF_CURRENCY = 'EUR'
+"Reference currency for the currency converter."
+
+
+logger = logging.getLogger(__name__)
+
+
+class CurrencyProxy:
+    """
+    Proxy for the currency converter.
+
+    This class is used to proxy the currency converter instance. This is to
+    ensure that the currency converter is only initialized when it is actually
+    used, and the used conversion list is the most up-to-date.
+    """
+
+    def __init__(self, app: Flask):
+        self._converter = None
+        self._app = app
+        self._converter_updated = 0
+        self._dataset_updated = 0
+
+    def get_currency_converter(self) -> CurrencyConverter:
+        """
+        Get a currency converter instance.
+
+        Automatically updates the currency converter if the dataset has been
+        updated.
+
+        Exceptions:
+            RuntimeError: If the currency file is not configured.
+            FileNotFoundError: If the currency file does not exist.
+
+        :return: A currency converter instance.
+        """
+
+        if not (conversion_file := self._app.config.get('CURRENCY_FILE')):
+            raise RuntimeError('Currency file not configured.')
+
+        # Initialize the currency converter if it has not been initialized yet,
+        # or if the dataset has been updated.
+        self._dataset_updated = Path(conversion_file).stat().st_mtime
+        if self._converter is None or self._dataset_updated > self._converter_updated:
+            logger.info("Initializing currency converter with file %s.", conversion_file)
+            self._converter = CurrencyConverter(
+                currency_file=conversion_file,
+                ref_currency=REF_CURRENCY,
+            )
+            self._converter_updated = self._dataset_updated
+
+        return self._converter
+
+    def __getattr__(self, name):
+        """
+        Proxy all other attributes to the currency converter.
+        """
+        return getattr(self.get_currency_converter(), name)
+
+
+def init_currency(app: Flask):
+    """
+    Initialize the currency module.
+
+    This function initializes the currency module, and registers the currency
+    converter as an extension.
+
+    :param app: The Flask application.
+    :return: None
+    """
+
+    # Set default currency file path
+    app.config.setdefault('CURRENCY_FILE', app.instance_path + '/currency.csv')
+
+    # Register the currency converter as an extension
+    app.extensions['currency_converter'] = CurrencyProxy(app)
+
+    # Register the currency converter as a template filter
+    app.add_template_filter(format_converted_currency, name='localcurrency')
+
+    app.cli.add_command(update_currency_rates)
+
+
+def format_converted_currency(value, currency=None, **kwargs):
+    """
+    Render a currency value in the preferred currency.
+
+    This function renders a currency value in the preferred currency for the
+    current locale. If the preferred currency is not the reference currency,
+    the value is converted to the preferred currency.
+    """
+
+    if currency is None:
+        currency = get_preferred_currency()
+
+    # Convert the value to the preferred currency
+    local_value = convert_currency(value, currency)
+
+    # Format the value
+    html = render_template("money-tag.html",
+                           base_amount=format_currency(value, currency=REF_CURRENCY, format_type='name', **kwargs),
+                           local_amount=format_currency(local_value, currency=currency, **kwargs))
+    return Markup(html)
+
+
+def convert_currency(value, currency=None, from_currency=REF_CURRENCY):
+    """
+    Convert a currency value to the preferred currency.
+
+    This function converts a currency value to the preferred currency for the
+    current locale. If the preferred currency is not the reference currency,
+    the value is converted to the preferred currency.
+    """
+
+    if currency != REF_CURRENCY:
+        return current_app.extensions['currency_converter'].convert(value, from_currency, currency)
+
+    return value
+
+
+def convert_from_currency(value, currency) -> Decimal:
+    """
+    Parses the localized currency value and converts it to the reference currency.
+    """
+
+    locale = get_locale()
+    amount = parse_decimal(value, locale=locale)
+
+    if currency != REF_CURRENCY:
+        amount = Decimal(current_app.extensions['currency_converter'].convert(amount, currency, REF_CURRENCY))
+
+    return amount
+
+
+def get_currencies():
+    """
+    Get the list of supported currencies.
+    """
+
+    return current_app.extensions['currency_converter'].currencies
+
+
+def get_preferred_currency():
+    """
+    Get the preferred currency.
+
+    This function returns the preferred currency for the current locale.
+
+    :return: The preferred currency.
+    """
+
+    if current_user.is_authenticated and current_user.currency:
+        return str(current_user.currency)
+
+    # Fall back to the default currency for the locale
+    if territory := get_locale().territory:
+        currency = get_territory_currencies(territory)[0]
+        if currency in get_currencies():
+            return currency
+        else:
+            logger.warning("Default currency %s is not supported, falling back to %s.", currency, REF_CURRENCY)
+
+    return REF_CURRENCY
+
+
+@click.command()
+def update_currency_rates():
+    """
+    Update currency file from the European Central Bank.
+
+    This command is meant to be run from the command line, and is not meant to be
+    used in the application:
+        $ flask update-currency-rates
+
+    :return: None
+    """
+    click.echo('Updating currency file from the European Central Bank...')
+    fetch_currency_file()
+    click.echo('Done.')
+
+
+def fetch_currency_file():
+    """
+    Fetch the currency file from the European Central Bank.
+
+    This function fetches the currency file from the European Central Bank, and
+    stores it in the configured currency file path.
+    """
+
+    from tempfile import NamedTemporaryFile  # pylint: disable=import-outside-toplevel
+
+    fd, _ = urllib.request.urlretrieve(SINGLE_DAY_ECB_URL)
+    with ZipFile(fd) as zf:
+        file_name = zf.namelist().pop()
+
+        # Create a temporary file to store the currency file, to avoid corrupting
+        # the existing file if the download fails, or while writing the file.
+
+        file_path = os.path.dirname(current_app.config['CURRENCY_FILE'])
+        if not os.path.exists(file_path):
+            os.makedirs(file_path)
+
+        with NamedTemporaryFile(dir=file_path, delete=False) as f:
+            f.write(zf.read(file_name))
+            f.flush()
+
+            # Move the temporary file to the configured currency file path
+            os.rename(f.name, current_app.config['CURRENCY_FILE'])

src/tjts5901/db.py

+import logging
+from os import environ
+
+from flask_mongoengine import MongoEngine
+
+db = MongoEngine()
+logger = logging.getLogger(__name__)
+
+
+def init_db(app):
+    """
+    Initialize the database connection.
+
+    Fetches the database connection string from the environment variable `MONGO_URL`
+    and, if present, sets the `MONGODB_SETTINGS` configuration variable to use it.
+    """
+
+    # To keep secrets private, we use environment variables to store the database connection string.
+    # `MONGO_URL` is expected to be a valid MongoDB connection string, see: blah blah blah
+    mongodb_url = environ.get("MONGO_URL")
+
+    if mongodb_url is not None:
+        app.config["MONGODB_SETTINGS"] = {
+            "host": mongodb_url,
+        }
+        logger.info("Database connection string found, using it.",
+                    # You can use the `extra` parameter to add extra information to the log message.
+                    # This is useful for debugging, but should be removed in production.
+                    extra={"MONGO_URL": mongodb_url} if app.debug else {})
+
+    else:
+        logger.warning("No database connection string found in env, using defaults.",
+                       extra={"MONGODB_SETTINGS": app.config.get("MONGODB_SETTINGS")} if app.debug else {})
+
+    db.init_app(app)
+

src/tjts5901/i18n.py

+"""
+Internationalisation and localisation support for the application.
+"""
+from enum import Enum
+import os
+from typing import List
+from flask_babel import Babel, get_locale as get_babel_locale
+from babel import Locale
+from babel import __version__ as babel_version
+from flask import (
+    Flask,
+    g,
+    request,
+    session,
+)
+
+from werkzeug.datastructures import LanguageAccept
+
+from flask_login import current_user
+
+import logging
+
+logger = logging.getLogger(__name__)
+
+
+class SupportedLocales(Enum):
+    """
+    Supported locales for the application.
+
+    The values are the locale identifiers used by the Babel library.
+    Order here determines the order in which the locales are presented to the
+    user, and the order in which the locales are tried when the user does not
+    specify a preferred locale.
+    """
+
+    FI = "fi_FI.UTF-8"
+    "Finnish (Finland)"
+
+    SV = "sv_SE.UTF-8"
+    "Swedish (Sweden)"
+
+    EN = "en_GB.UTF-8"
+    "English (United Kingdom)"
+
+    # EN_US = "en_US.UTF-8"
+    # "English (United States)"
+
+    TLH = "tlh"
+    "Klingon"
+
+
+TIMEZONES = {
+    """
+    Timezones for supported locales.
+
+    The values are the timezone identifiers used by the Babel library.
+    This approach doesnt work for countries that have multiple timezones, like
+    the US.
+    """
+    "fi_FI": "Europe/Helsinki",
+    "sv_SE": "Europe/Stockholm",
+    "en_GB": "Europe/London",
+    "tlh": "America/New_York",
+}
+
+def init_babel(flask_app: Flask):
+    """
+    Initialize the Flask-Babel extension.
+    """
+
+    # Monkeypatch klingon support into babel
+    # Klingon reverts to English
+    hack_babel_core_to_support_custom_locales({"tlh": "en"})
+
+    # Configure the Flask-Babel extension.
+    # Try setting the default locale from underlying OS. Falls back into English.
+    system_language = Locale.default().language
+    translation_dir = os.path.join(os.path.dirname(__file__), "translations")
+    flask_app.config.setdefault("BABEL_TRANSLATION_DIRECTORIES", translation_dir)
+    flask_app.config.setdefault("BABEL_DEFAULT_LOCALE", system_language)
+
+    babel = Babel(flask_app, locale_selector=get_locale, timezone_selector=get_timezone)
+
+    # Register `locales` as jinja variable to be used in templates. Uses the
+    # `Locale` class from the Babel library, so that the locale names can be
+    # translated.
+    locales = {}
+    for locale in SupportedLocales:
+        locales[locale.value] = Locale.parse(locale.value)
+
+    flask_app.jinja_env.globals.update(locales=locales)
+    # Register `get_locale` as jinja function to be used in templates
+    flask_app.jinja_env.globals.update(get_locale=get_babel_locale)
+
+    # If url contains locale parameter, set it as default in session
+    @flask_app.before_request
+    def set_locale():
+        if request.endpoint != "static":
+            if locale := request.args.get('locale'):
+                if locale in (str(l) for l in locales.values()):
+                    logger.debug("Setting locale %s from URL.", locale)
+                    session['locale'] = locale
+                else:
+                    logger.warning("Locale %s not supported.", locale)
+
+    logger.info("Initialized Flask-Babel extension %s.", babel_version,
+                extra=flask_app.config.get_namespace("BABEL_"))
+
+    return babel
+
+
+def hack_babel_core_to_support_custom_locales(custom_locales: dict):
+    """ Hack Babel core to make it support custom locale names
+
+    Based on : https://github.com/python-babel/babel/issues/454
+
+    Patch mechanism provided by @kolypto
+
+    Args:
+        custom_locales: Mapping from { custom name => ordinary name }
+    """
+    from babel.core import get_global
+
+    # In order for Babel to know "en_CUSTOM", we have to hack its database and put our custom
+    # locale names there.
+    # This database is pickle-loaded from a .dat file and cached, so we only have to do it once.
+    db = get_global('likely_subtags')
+    for custom_name in custom_locales:
+        db[custom_name] = custom_name
+
+    # Also, monkey-patch the exists() and load() functions that load locale data from 'babel/locale-data'
+    import babel.localedata
+
+    # Originals
+    o_exists, o_load, o_parse_locale = babel.localedata.exists, babel.localedata.load, babel.core.parse_locale
+
+    # Definitions
+    def exists(name):
+        # Convert custom names to normalized names
+        name = custom_locales.get(name, name)
+        return o_exists(name)
+
+    def load(name, merge_inherited=True):
+        # Convert custom names to normalized names
+        original_name = custom_locales.get(name, name)
+        l_data = o_load(original_name, merge_inherited)
+        l_data['languages']['tlh'] = 'Klingon'
+
+        l_data.update({
+            'locale_id': name,
+        })
+        return l_data
+
+    # Definitions
+    def parse_locale(name, sep='_'):
+        # Convert custom names to normalized names
+        name = custom_locales.get(name, name)
+        l_data = o_parse_locale(name, sep)
+        return l_data
+
+    # Make sure we do not patch twice
+    if o_exists.__module__ != __name__:
+        babel.localedata.exists = exists
+        babel.localedata.load = load
+
+    # if o_parse_locale.__module__ != __name__:
+    #    babel.core.parse_locale = parse_locale
+
+    # See that they actually exist
+    for normalized_name in custom_locales.values():
+        assert o_exists(normalized_name)
+
+
+def get_locale():
+    """
+    Get the locale for user.
+
+    Looks at the user model for the user's preferred locale. If the user has not
+    set a preferred locale, check the browser's Accept-Language header. If the
+    browser does not specify a preferred locale, use the default locale.
+
+    todo: What happens if the user's preferred locale support is dropped from
+    todo: the application?
+
+    :return: Suitable locale for the user.
+    """
+
+    # if a locale was stored in the session, use that
+    if locale := session.get('locale'):
+        logger.debug("Setting locale %s from session.", locale)
+        return locale
+
+    # if a user is logged in, use the locale from the user settings
+    if current_user.is_authenticated and current_user.locale:
+        logger.debug("Using locale %s from user settings.", current_user.locale)
+        return current_user.locale
+
+    # otherwise try to guess the language from the user accept header the
+    # browser transmits.
+
+    # The Accept-Language header is a list of languages the user prefers,
+    # ordered by preference. The first language is the most preferred.
+    # The language is specified as a language tag, which is a combination of
+    # a language code and a country code, separated by a hyphen.
+    # For example, en-GB is English (United Kingdom).
+    # The language code is a two-letter code, and the country code is a
+    # two-letter code, or a three-digit number. The country code is optional.
+    # For example, en is English (no country specified), and en-US is English
+
+    # Convert the Enum of supported locales into a list of language tags.
+    # Fancy way: locales_to_try = [locale.value for locale in SupportedLocales]
+    locales_to_try: List[str] = list()
+    for locale in SupportedLocales:
+        locales_to_try.append(str(locale.value))
+
+    # Get the best match for the Accept-Language header.
+    locale = request.accept_languages.best_match(locales_to_try)
+
+    logger.debug("Best match for Accept-Language header (%s) is %s.",
+                 request.accept_languages, locale)
+
+    return locale
+
+
+def get_timezone():
+    """
+    Get the timezone for user.
+
+    Looks at the user model for the user's preferred timezone. If the user has
+    not set a preferred timezone, use the default timezone.
+    """
+
+    # if a user is logged in, use the timezone from the user settings
+    if current_user.is_authenticated and current_user.timezone:
+        logger.debug("Using locale %s from user settings.", current_user.timezone)
+        return current_user.timezone
+
+    # Try detecting the timezone from the user's locale.
+    locale = get_locale()
+    choises = [(k, 1) for k in TIMEZONES.keys()]
+
+    # Use the best_match method from the LanguageAccept class to get the best
+    # match for the user's locale.
+    best_match = LanguageAccept(choises).best_match([locale])
+    if best_match:
+        logger.debug("Guessing timezone %s from locale %s.", TIMEZONES[best_match], locale)
+        return TIMEZONES[best_match]

src/tjts5901/items.py

+from datetime import datetime, timedelta
+import logging
+from typing import Optional
+from flask import (
+    Blueprint, flash, redirect, render_template, request, url_for, jsonify, current_app
+)
+from flask_babel import _, get_locale, lazy_gettext
+from werkzeug.exceptions import abort
+
+from markupsafe import Markup
+
+from .auth import login_required, current_user
+from .models import Bid, Item
+from .currency import (
+    convert_currency,
+    format_converted_currency,
+    convert_from_currency,
+    get_currencies,
+    get_preferred_currency,
+    REF_CURRENCY,
+)
+from .notification import send_notification
+
+bp = Blueprint('items', __name__)
+api = Blueprint('api_items', __name__, url_prefix='/api/items')
+
+logger = logging.getLogger(__name__)
+
+MIN_BID_INCREMENT = 1
+
+
+def get_item(id):
+    try:
+        item = Item.objects.get_or_404(id=id)
+    except Exception as exc:
+        print("Error getting item:", exc)
+        abort(404)
+
+    if item.seller == current_user:
+        return item
+    
+    abort(403)
+
+
+def get_winning_bid(item: Item) -> Optional[Bid]:
+    """
+    Return the (currently) winning bid for the given item.
+
+    If there are no bids, or the item is not yet closed, return None.
+
+    :param item: The item to get the winning bid for.
+    :return: The winning bid, or None.
+    """
+
+    winning_bid = None
+
+    # If the item is closed, return the winning bid
+    if item.closed and item.winning_bid:
+        return item.winning_bid
+
+    # Sanity check: if the item is not closed, it should not have a winning bid
+    assert not item.closed or not (not item.closed and winning_bid), "Item is not closed, but has a winning bid"
+
+    try:
+        # Get the highest bid that was placed before the item closed
+        winning_bid = Bid.objects(item=item) \
+            .filter(created_at__lt=item.closes_at) \
+            .order_by('-amount') \
+            .first()
+    except Exception as exc:
+        logger.warning("Error getting winning bid: %s", exc, exc_info=True, extra={
+            'item_id': item.id,
+        })
+
+    return winning_bid
+
+
+def get_item_price(item: Item) -> int:
+    """
+    Return the current price of the given item.
+
+    If there are no bids, return the starting bid.
+
+    :param item: The item to get the price for.
+    :return: The current price.
+    """
+
+    winning_bid = get_winning_bid(item)
+    if winning_bid:
+        return winning_bid.amount + MIN_BID_INCREMENT
+    else:
+        return item.starting_bid
+
+
+def handle_item_closing(item):
+    """
+    Handle the closing of an item.
+
+    Checks if the item is not closed yet, but should be closed now. If so, 
+    closes the item, and send notifications to the seller and the buyer.
+
+    :param item: The item to handle.
+    """
+    # Handle the closing of an item
+    if not item.is_open and not item.closed:
+        logger.info("Closing item %r (%s)", item.title, item.id, extra={
+            'item_id': item.id,
+            'item_title': item.title,
+            'item_closes_at': item.closes_at,
+        })
+
+        # Get the winning bid
+        winning_bid = get_winning_bid(item)
+        if winning_bid:
+            item.winning_bid = winning_bid
+
+            # Send a notifications to the seller and the buyer
+            # lazy_gettext() is used to delay the translation until the message is sent
+            # Markup.escape() is used to escape strings, to prevent XSS attacks
+            send_notification(
+                item.seller,
+                title=lazy_gettext("Your item was sold"),
+                message=lazy_gettext("Your item <em>%(title)s</em> was sold to %(buyer)s for %(price)s.",
+                                     title=Markup.escape(item.title),
+                                     buyer=Markup.escape(winning_bid.bidder.email),
+                                     price=Markup.escape(winning_bid.amount)),
+            )
+            send_notification(
+                winning_bid.bidder,
+                title=lazy_gettext("You won an item"),
+                message=lazy_gettext("You won the item <em>%(title)s</em> for %(price)s.",
+                                     title=Markup.escape(item.title),
+                                     price=Markup.escape(winning_bid.amount)),
+            )
+
+        else:
+            # If there is no winning bid, send a notification to the seller
+            send_notification(
+                item.seller,
+                title=lazy_gettext("Your item was not sold"),
+                message=lazy_gettext("Your item <em>%(title)s</em> was not sold.",
+                                     title=Markup.escape(item.title)),
+            )
+
+        # Close the item
+        item.closed = True
+        item.save()
+
+
+@bp.route("/", defaults={'page': 1})
+@bp.route("/items/<int:page>")
+def index(page=1):
+    """
+    Index page for items on sale.
+
+    Lists only items that are currently sale, with pagination.
+    """
+
+    # Function used on propaedeutic
+    # items = Item.objects.all()
+
+    # Fetch items that are on sale currently, and paginate
+    # See: http://docs.mongoengine.org/projects/flask-mongoengine/en/latest/custom_queryset.html
+    items = Item.objects.filter(closes_at__gt=datetime.utcnow()) \
+        .order_by('-closes_at') \
+        .paginate(page=page, per_page=10)
+
+    return render_template('items/index.html',
+                           items=items)
+
+
+@bp.route('/sell', methods=('GET', 'POST'))
+@login_required
+def sell():
+    if request.method == 'POST':
+        title = request.form['title']
+        description = request.form['description']
+        
+        currency = request.form.get('currency', REF_CURRENCY)
+        starting_bid = convert_from_currency(request.form['starting_bid'], currency)
+
+        error = None
+
+        if not title:
+            error = 'Title is required.'
+        if not starting_bid or starting_bid < 1:
+            error = Markup(_("Starting bid must be greater than %(amount)s.", amount=format_converted_currency(1, currency)))
+
+        if error is None:
+            try:
+                sale_length = timedelta(days=1)
+                if current_app.config['DEBUG'] and request.form.get("flash-sale"):
+                    sale_length = timedelta(seconds=20)
+
+                item = Item(
+                    title=title,
+                    description=description,
+                    starting_bid=starting_bid,
+                    seller=current_user,
+                    closes_at=datetime.utcnow() + sale_length,
+                )
+                item.save()
+                flash(_('Item listed successfully!'))
+
+            except Exception as exc:
+                error = _("Error creating item: %(exc)s", exc=exc)
+                logger.warning("Error creating item: %s", exc, exc_info=True, extra={
+                    'title': title,
+                    'description': description,
+                    'starting_bid': starting_bid,
+                })
+            else:
+                return redirect(url_for('items.index'))
+
+        print(error)
+        flash(error, category='error')
+
+    # Get the list of currencies, and map them to their localized names
+    currencies = {}
+    names = get_locale().currencies
+    for currency in get_currencies():
+        currencies[currency] = names.get(currency, currency)
+
+    return render_template('items/sell.html', currencies=currencies, default_currency=get_preferred_currency())
+
+
+@bp.route('/item/<id>')
+def view(id):
+    """
+    Item view page.
+
+    Displays the item details, and a form to place a bid.
+    """
+
+    item = Item.objects.get_or_404(id=id)
+
+    # !!! This is disabled as it might cause race conditions
+    # !!! if multiple users are accessing the same item at the same time
+    # Check if the item is closed, and handle it if so.
+    #handle_item_closing(item)
+
+    # Set the minumum price for the bid form from the current winning bid
+    winning_bid = get_winning_bid(item)
+    min_bid = get_item_price(item)
+
+    local_currency = get_preferred_currency()
+    local_min_bid = convert_currency(min_bid, local_currency)
+
+    if item.closes_at < datetime.utcnow():
+        if winning_bid and winning_bid.bidder == current_user:
+            flash(_("Congratulations! You won the auction!"), "success")
+        else:
+            flash(_("This item is no longer on sale."))
+    elif item.closes_at < datetime.utcnow() + timedelta(hours=1):
+        # Dark pattern to show enticing message to user
+        flash(_("This item is closing soon! Act now! Now! Now!"))
+
+    return render_template('items/view.html',
+                           item=item, min_bid=min_bid,
+                           local_min_bid=local_min_bid,
+                           local_currency=local_currency)
+
+
+@bp.route('/item/<id>/update', methods=('GET', 'POST'))
+@login_required
+def update(id):
+    item = get_item(id)
+
+    if request.method == 'POST':
+        title = request.form['title']
+        description = request.form['description']
+        error = None
+
+        if not title:
+            error = _('Title is required.')
+
+        try:
+            item.title = title
+            item.description = description
+            item.save()
+        except Exception as exc:
+            error = _("Error updating item: %(exc)s", exc=exc)
+            logger.warning("Error updating item: %s", exc, exc_info=True, extra={
+                'item_id': item.id,
+            })
+        else:
+            flash(_("Item updated successfully!"))
+            return redirect(url_for('items.index'))
+
+        print(error)
+        flash(error, category='error')
+
+    return render_template('items/update.html', item=item)
+
+
+@bp.route('/item/<id>/delete', methods=('POST',))
+@login_required
+def delete(id):
+    item = get_item(id)
+    try:
+        item.delete()
+    except Exception as exc:
+        logger.warning("Error deleting item: %s", exc, exc_info=True, extra={
+            'item_id': item.id,
+        })
+        flash(_("Error deleting item: %(exc)s", exc=exc), category='error')
+    else:
+        flash(_("Item deleted successfully!"))
+    return redirect(url_for('items.index'))
+
+
+@bp.route('/item/<id>/bid', methods=('POST',))
+@login_required
+def bid(id):
+    """
+    Bid on an item.
+
+    If the bid is valid, create a new bid and redirect to the item view page.
+    Otherwise, display an error message and redirect back to the item view page.
+
+    :param id: The id of the item to bid on.
+    :return: A redirect to the item view page.
+    """
+
+    item = Item.objects.get_or_404(id=id)
+    min_amount = get_item_price(item)
+
+    local_amount = request.form['amount']
+    currency = request.form.get('currency', REF_CURRENCY)
+
+    amount = convert_from_currency(local_amount, currency)
+
+    if amount < min_amount:
+        flash(_("Bid must be at least %(min_amount)s", min_amount=format_converted_currency(min_amount)))
+        return redirect(url_for('items.view', id=id))
+
+    if item.closes_at < datetime.utcnow():
+        flash("This item is no longer on sale.")
+        return redirect(url_for('items.view', id=id))
+
+    try:
+        # Notice: if you have integrated the flask-login extension, use current_user
+        # instead of g.user
+        bid = Bid(
+            item=item,
+            bidder=current_user,
+            amount=amount,
+        )
+        bid.save()
+    except Exception as exc:
+        flash(_("Error placing bid: %(exc)s", exc=exc))
+    else:
+        flash(_("Bid placed successfully!"))
+
+    return redirect(url_for('items.view', id=id))
+
+
+@api.route('<id>/bids', methods=('GET',))
+@login_required
+def api_item_bids(id):
+    """
+    Get the bids for an item.
+
+    :param id: The id of the item to get bids for.
+    :return: A JSON response containing the bids.
+    """
+
+    item = Item.objects.get_or_404(id=id)
+    bids = []
+    for bid in Bid.objects(item=item).order_by('-amount'):
+        bids.append(bid.to_json())
+
+    return jsonify({
+        'success': True,
+        'bids': bids
+    })
+
+@api.route('<id>/bids', methods=('POST',))
+@login_required
+def api_item_place_bid(id):
+    """
+    Place a bid on an item.
+
+    If the bid is valid, create a new bid and return the bid.
+    Otherwise, return an error message.
+    
+    Only accepts `REF_CURRENCY` bids.
+
+    :param id: The id of the item to bid on.
+    :return: A JSON response containing the bid.
+    """
+
+    item = Item.objects.get_or_404(id=id)
+    min_amount = get_item_price(item)
+
+    try:
+        amount = int(request.form['amount'])
+    except KeyError:
+        return jsonify({
+            'success': False,
+            'error': _("Missing required argument %(argname)s", argname='amount')
+        })
+    except ValueError:
+        return jsonify({
+            'success': False,
+            'error': _("Invalid value for argument %(argname)s", argname='amount')
+        })
+    except Exception as exc:
+        return jsonify({
+            'success': False,
+            'error': _("Error parsing argument %(argname)s: %(exc)s", argname='amount', exc=exc)
+        })
+
+    if amount < min_amount:
+        return jsonify({
+            'success': False,
+            'error': _("Bid must be at least %(min_amount)s", min_amount=min_amount)
+        })
+
+    if item.closes_at < datetime.utcnow():
+        return jsonify({
+            'success': False,
+            'error': _("This item is no longer on sale.")
+        })
+
+    try:
+        bid = Bid(
+            item=item,
+            bidder=current_user,
+            amount=amount,
+        )
+        bid.save()
+    except Exception as exc:
+        logger.error("Error placing bid: %s", exc, exc_info=True, extra={
+            'item_id': item.id,
+            'bidder_id': current_user.id,
+            'amount': amount,
+        })
+
+        return jsonify({
+            'success': False,
+            'error': _("Error placing bid: %(exc)s", exc=exc)
+        })
+
+    return jsonify({
+        'success': True,
+        'bid': bid.to_mongo().to_dict()
+    })

src/tjts5901/logging.py

+"""
+==============
+Logging module
+==============
+
+In this module we'll create a new :class:`~Logger` interface, using pythons inbuild :module:`logging` module.
+By default flask sends messages to stdout.
+
+To use this module, import it into your application and call :func:`~init_logging` function:
+    >>> from tjts5901.logging import init_logging
+    >>> init_logging(app)
+
+To use logging in your application, import the logger instance, and use it as follows:
+    >>> import logging
+    >>> logger = logging.getLogger(__name__)
+    >>> logger.info("Hello world!")
+
+"""
+
+
+import logging
+from os import environ
+
+import sentry_sdk
+from flask import Flask
+from flask.logging import default_handler as flask_handler
+from sentry_sdk.integrations.flask import FlaskIntegration
+from sentry_sdk.integrations.pymongo import PyMongoIntegration
+
+from .utils import get_version
+
+def init_logging(app: Flask):
+    """
+    Integrate our own logging interface into application.
+
+    To bind logger into your application instance use::
+        >>> init_logging(app)
+
+    :param app: :class:`~Flask` instance to use as logging basis.
+    """
+
+    # Setup own logger instance. Usually you'll see something like
+    # >>> logger = logging.getLogger(__name__)
+    # where `__name__` reflects the package name, which is usually `"__main__"`,
+    # or in this exact case `tjts5901.logging`. I'll rather define static name.
+    # To get access to your logger in outside of module scope you can then
+    # use the same syntax as follows.
+    logger = logging.getLogger("tjts5901")
+
+    # If flask is running in debug mode, set our own handler to log also debug
+    # messages.
+    if app.config.get("DEBUG"):
+        logger.setLevel(level=logging.DEBUG)
+
+    # Add flask default logging handler as one of our target handlers.
+    # When changes to flask logging handler is made, our logging handler
+    # adapts automatically. Logging pipeline:
+    # our appcode -> our logger -> flask handler -> ????
+    logger.addHandler(flask_handler)
+
+    logger.debug("TJTS5901 Logger initialised.")
+
+    # Try to get enviroment name from different sources
+    if enviroment := environ.get('CI_ENVIRONMENT_NAME'):
+        enviroment = enviroment.lower()
+    elif app.testing:
+        enviroment = "testing"
+    elif app.debug:
+        enviroment = "development"
+
+    # Populate config with environment variables for sentry logging
+    app.config.setdefault('SENTRY_DSN', environ.get('SENTRY_DSN'))
+    app.config.setdefault('SENTRY_ENVIRONMENT', enviroment)
+    app.config.setdefault('CI_COMMIT_SHA', environ.get('CI_COMMIT_SHA'))
+
+    # Setup sentry logging
+    sentry_dsn = app.config.get("SENTRY_DSN")
+    release = app.config.get("CI_COMMIT_SHA", get_version() or "dev")
+    enviroment = app.config.get("SENTRY_ENVIRONMENT", "production")
+
+    if sentry_dsn:
+        sentry = sentry_sdk.init(
+            dsn=sentry_dsn,
+            integrations=[
+                # Flask integration
+                FlaskIntegration(),
+
+                # Mongo integration. Mongoengine uses pymongo, so we need to
+                # integrate pymongo.
+                PyMongoIntegration(),
+
+                # Sentry will automatically pick up the logging module.
+                #LoggingIntegration(level=logging.INFO, event_level=logging.ERROR),
+            ],
+
+            # Set traces_sample_rate to 1.0 to capture 100%
+            # of transactions for performance monitoring.
+            # We recommend adjusting this value in production.
+            traces_sample_rate=1.0,
+
+            # Set sentry debug mode to true if flask is running in debug mode.
+            #debug=bool(app.debug),
+
+            # By default the SDK will try to use the SENTRY_RELEASE
+            # environment variable, or infer a git commit
+            # SHA as release, however you may want to set
+            # something more human-readable.
+            release=release,
+            environment=enviroment,
+        )
+
+        app.config.setdefault("SENTRY_RELEASE", sentry._client.options["release"])
+        logger.info("Sentry logging enabled.", extra={"SENTRY_DSN": sentry_dsn})
+
+    else:
+        logger.warning("Sentry DSN not found. Sentry logging disabled.")

src/tjts5901/models.py

+from datetime import datetime
+from secrets import token_urlsafe
+from urllib.parse import urlencode
+from flask import url_for
+from markupsafe import Markup
+
+from .db import db
+
+from mongoengine import (
+    StringField,
+    IntField,
+    ReferenceField,
+    DateTimeField,
+    EmailField,
+    BooleanField,
+    EnumField,
+)
+
+from mongoengine.queryset import CASCADE
+
+from flask_login import UserMixin
+from bson import ObjectId
+
+from .i18n import SupportedLocales
+
+
+class User(UserMixin, db.Document):
+    """
+    Model representing a user of the auction site.
+    """
+    id: ObjectId
+
+    email = EmailField(required=True, unique=True)
+    "The user's email address."
+
+    password = StringField(required=True)
+
+    locale = StringField(default=SupportedLocales.EN.value)
+    currency = StringField(max_length=3)
+    "The user's preferred currency."
+
+    timezone = StringField(max_length=50)
+
+    created_at = DateTimeField(required=True, default=datetime.utcnow)
+
+    is_disabled = BooleanField(default=False)
+    "Whether the user is disabled."
+
+    @property
+    def image_url(self) -> str:
+        """
+        Return the URL of the user's avatar.
+        """
+        import hashlib
+
+        digest = hashlib.md5(self.email.lower().encode("utf-8")).hexdigest()
+        default = url_for("static", filename="img/default-profile.png", _external=True)
+        params = urlencode({"s": 200, "d": default})
+        return f"https://www.gravatar.com/avatar/{digest}?{params}"
+
+    @property
+    def is_active(self) -> bool:
+        """
+        Return whether the user is active.
+
+        This is used by Flask-Login to determine whether the user is
+        allowed to log in.
+        """
+        return not self.is_disabled
+
+    def get_id(self) -> str:
+        """
+        Return the user's id as a string.
+        """
+        return str(self.id)
+
+    def __html__(self) -> str:
+        """
+        Return a html representation of the user.
+        """
+
+        url = url_for("auth.profile", email=self.email)
+        username = self.email.split("@")[0]
+        safe_username = Markup.escape(username)
+        return f"<a href=\"{url}\" class=\"profile-link\">@{safe_username}</a>"
+
+
+class Item(db.Document):
+    """
+    A model for items that are listed on the auction site.
+    """
+
+    # Create index for sorting items by closing date
+    meta = {"indexes": [
+        {"fields": [
+            "closes_at",
+        ]}
+    ]}
+
+    title = StringField(max_length=100, required=True)
+    description = StringField(max_length=2000, required=True)
+
+    starting_bid = IntField(required=True, min_value=0)
+
+    seller = ReferenceField(User, required=True)
+    winning_bid = ReferenceField("Bid")
+    closed = BooleanField(default=False)
+    "Whether the item has been closed."
+
+    created_at = DateTimeField(required=True, default=datetime.utcnow)
+    closes_at = DateTimeField()
+
+    @property
+    def is_open(self) -> bool:
+        """
+        Return whether the item is open for bidding.
+        """
+        if self.closed:
+            return False
+        return self.closes_at > datetime.utcnow()
+
+
+class Bid(db.Document):
+    """
+    A model for bids on items.
+    """
+
+    meta = {"indexes": [
+        {"fields": [
+            "amount",
+            "item",
+            "created_at",
+        ]}
+    ]}
+
+    amount = IntField(required=True, min_value=0)
+    "Indicates the value of the bid."
+
+    bidder = ReferenceField(User, required=True)
+    "User who placed the bid."
+
+    item = ReferenceField(Item, required=True)
+    "Item that the bid is for."
+
+    created_at = DateTimeField(required=True, default=datetime.utcnow)
+    "Date and time that the bid was placed."
+
+
+class AccessToken(db.Document):
+    """
+    Access token for a user.
+
+    This is used to authenticate API requests.
+    """
+
+    meta = {"indexes": [
+        {"fields": [
+            "token",
+            "user",
+            "expires",
+        ]}
+    ]}
+
+    name = StringField(max_length=100, required=True)
+    "Human-readable name for the token."
+
+    user = ReferenceField(User, required=True, reverse_delete_rule=CASCADE)
+    "User that the token is for."
+
+    token = StringField(required=True, unique=True, default=token_urlsafe)
+    "The token string."
+
+    last_used_at = DateTimeField(required=False)
+    "Date and time that the token was last used."
+
+    created_at = DateTimeField(required=True, default=datetime.utcnow)
+    "Date and time that the token was created."
+
+    expires = DateTimeField(required=False)
+    "Date and time that the token expires."
+
+
+class Notification(db.Document):
+    """
+    Represents a message between two users, or a message to a user from
+    the system.
+    """
+
+    meta = {"indexes": [
+        {"fields": [
+            "user",
+            "read_at",
+            "created_at",
+        ]}
+    ]}
+
+    id: ObjectId
+
+    user = ReferenceField(User, required=True, reverse_delete_rule=CASCADE)
+
+    category = StringField(max_length=100, default="message")
+    message = StringField(required=True)
+    title = StringField(max_length=120)
+
+    created_at = DateTimeField(required=True, default=datetime.utcnow)
+    read_at = DateTimeField(required=False)

src/tjts5901/notification.py

+"""
+This module provides a way to send notifications to users.
+"""
+
+import dataclasses
+from datetime import datetime
+import logging
+
+from flask import get_flashed_messages, jsonify, Blueprint
+from flask_login import current_user, login_required
+from flask_babel import force_locale, lazy_gettext
+
+from .models import Notification, User
+
+bp = Blueprint('notification', __name__, url_prefix='/')
+
+logger = logging.getLogger(__name__)
+
+@dataclasses.dataclass
+class Message:
+    """
+    Represents a message to be displayed to the user.
+    """
+    message: str
+    category: str
+
+    # Additional field for the template
+    title: str = lazy_gettext("Message")
+
+    created_at: datetime = dataclasses.field(default_factory=datetime.utcnow)
+
+
+def init_notification(app):
+    """
+    Initialize the notifications module.
+    """
+    app.register_blueprint(bp)
+    app.jinja_env.globals.update(get_notifications=get_notifications)
+
+
+def send_notification(user, message, category="message", title=None):
+    """
+    Send a notification to the given user.
+
+    :param user: The user to send the message to.
+    :param subject: The subject of the message.
+    :param message: The message to send.
+    """
+
+    # Change the locale to the message recipient locale.
+    with force_locale(user.locale):
+        notification = Notification(
+            user=user,
+            message=str(message),
+            category=category,
+            title=str(title),
+        )
+        notification.save()
+
+
+def get_notifications(user: User = current_user) -> list[Message]:
+    """
+    Get the messages for the given user.
+
+    Flash messages are returned first, followed by database messages.
+    Messages are marked as read when they are retrieved.
+
+    Notice: the listing query might not return recently added messages.
+
+    :param user: The user to get the messages for.
+    :return: A list of messages.
+    """
+
+    messages = []
+
+    # Get the flash messages first
+    for category, message in get_flashed_messages(with_categories=True):
+        messages.append(Message(message, category))
+
+    if user is None or not user.is_authenticated:
+        logger.debug("User is not authenticated, returning flash messages.")
+        return messages
+
+    # Get the database messages
+    notifications = Notification.objects(user=user, read_at=None).order_by('-created_at').all()
+
+    for notification in notifications:
+        messages.append(Message(notification.message, notification.category, notification.title))
+
+    # Mark the messages as read
+    notifications.update(read_at=datetime.utcnow())
+
+    return messages
+
+
+@bp.route('/notifications.json', methods=('GET',))
+@login_required
+def user_notifications():
+    """
+    Show the form to send a message to the given user.
+    """
+
+    user = current_user
+
+    # Convert the notifications to a list of dictionaries
+    notifications = []
+    for notification in get_notifications(user):
+        notifications.append(dataclasses.asdict(notification))
+
+    return jsonify({
+        "success": True,
+        "notifications": notifications,
+    })

src/tjts5901/scheduler.py

+"""
+This module contains the APScheduler extension.
+
+This extension is used to schedule background tasks.
+"""
+
+from datetime import datetime, timedelta
+import logging
+from random import randint
+
+from flask_apscheduler import APScheduler
+from apscheduler.schedulers import SchedulerAlreadyRunningError
+from mongoengine import signals, Q
+
+from .models import Item
+from .items import handle_item_closing
+
+logger = logging.getLogger(__name__)
+
+# Having the scheduler as a global variable is not ideal, but it's the easiest
+# way to make it accessible.
+scheduler = APScheduler()
+
+
+def init_scheduler(app):
+    """
+    Initialize the APScheduler extension.
+
+    This function is meant to be called from the create_app() function.
+    """
+
+    try:
+
+        scheduler.init_app(app)
+
+        # Due to the scheduler being utilised as global variable, check if
+        # the scheduler is already running. If it is, then it means that the
+        # scheduler has already been initialised.
+        if not scheduler.running and not app.config.get('TESTING'):
+
+            # Add a signal handler to schedule a task to close the item when the auction
+            # ends.
+            signals.post_save.connect(_schedule_item_closing_task, sender=Item)
+
+            # Add a batch task to close expired bids every 15 minutes. This is to ensure
+            # that the bids are closed even if the server is restarted.
+            scheduler.add_job(trigger='interval', minutes=15,
+                            func=_close_items,
+                            id='close-items')
+
+            # Add a task to update the currency rates from the European Central Bank every
+            # day at random time between 5:00 and 5:59.
+            scheduler.add_job(trigger='cron', hour=5, minute=randint(0, 59),
+                            func=_update_currency_rates,
+                            id='update-currency-rates')
+
+            with app.app_context():
+                scheduler.start()
+                logger.debug('APScheduler started')
+
+    except SchedulerAlreadyRunningError:
+        logger.debug('APScheduler already running')
+
+    except Exception as exc:
+        logger.exception("Failed to initialize APScheduler: %s", exc)
+    return app
+
+
+def _handle_item_closing(item_id):
+    """
+    Handle the closing of an item.
+
+    This function is meant to be run by the APScheduler, and is not meant to be
+    called directly.
+    """
+
+    with scheduler.app.app_context():
+        item = Item.objects.get(id=item_id)
+        handle_item_closing(item)
+
+
+# Even as this is named function, it's used as a closure, so it can access
+# the scheduler variable.
+def _schedule_item_closing_task(sender, document, **kwargs):  # pylint: disable=unused-argument
+    """
+    Schedule a task to close the item when the auction ends.
+
+    This function is meant to be connected to the post_save signal of the Item
+    model.
+    """
+
+    if not document.closes_at:
+        # The item does not have an auction end time, so there is no need to
+        # schedule a task to close it.
+        logger.debug("Not scheduling closing, as item %s does not have an auction end time", document.id)
+        return
+
+    if document.closed:
+        # The item is already closed, so there is no need to schedule a task to
+        # close it.
+        return
+
+    logger.debug('Scheduling task to close item %s', document.id)
+    scheduler.add_job(
+        func=_handle_item_closing,
+        args=(document.id,),
+        trigger='date',
+        run_date=document.closes_at + timedelta(seconds=1),
+        id=f'close-item-{document.id}',
+    )
+
+
+def _close_items():
+    """
+    Close expired bids.
+
+    This function is meant to be run by the APScheduler, and is not meant to be
+    called directly.
+    """
+    with scheduler.app.app_context():
+        logger.info("Running scheduled task 'close-items'")
+
+        # Get items that are past the closing date, and are not already closed
+        closes_before = datetime.utcnow() + timedelta(seconds=2)
+        items = Item.objects(Q(closed=None) | Q(closed=False), closes_at__lt=closes_before).all()
+        logger.debug("Closing %d items", len(items))
+
+        # Close each item
+        for item in items:
+            try:
+                # Make sure item is not already closed
+                if item.closed:
+                    continue
+
+                handle_item_closing(item)
+
+            except Exception as exc:
+                logger.error("Error closing items: %s", exc, exc_info=True, extra={
+                    'item_id': item.id,
+                })
+
+
+def _update_currency_rates():
+    """
+    Update the currency rates from the European Central Bank.
+
+    This function is meant to be run by the APScheduler, and is not meant to be
+    called directly.
+    """
+    from .currency import fetch_currency_file
+    with scheduler.app.app_context():
+        logger.debug("Running scheduled task 'update-currency-rates'")
+        fetch_currency_file()