Refactor the big monster into pieces

refactor-monster-pieces-better.py

def create_order(products, user):
    connection = db_connection()
    connection.begin_transaction()

    try:
        process_order(connection, products, user)
    except Exception as e:
        connection.rollback()
        message = 'Order failed : %s' % e
    else:
        connection.commit()
        message = 'Order successful!'
    finally:
        connection.close()

    send_email(user, message)


def process_order(connection, products, user):
    amount = calculate_amount(products)
    order = new_order(connection, amount, user)
    add_items(connection, order)
    create_payment(connection, amount, user)


def calculate_amount(products):
    amount = Decimal('0')
    for product in products:
        amount += product.price
    return amount


def new_order(connection, amount, user):
    order_repository = order_repository_for(connection)
    order = order_repository.create()
    order.amount = amount
    order.user = user
    order.date = today()
    return order


def add_items(connection, order):
    item_repository = item_repository_for(connection)
    for product in products:
        item = item_repository.create()
        item.name = product.name
        item.price = product.price
        item.date = today()
        item.order = order
        item_repository.add(item)


def create_payment(connection, amount, user):
    payment_repository = payment_repository_for(connection)
    payment = payment_repository.create()
    payment.amount = amount
    payment.user = user

    authorized = authorize(payment)

    payment.transaction_id = authorized.transaction_id
    payment_repository.add(payment)


def authorize(payment):
    authorized = payment_gateway.authorize(payment)
    if not authorized:
        raise Exception('You cannot buy that!')
    return authorized


def send_email(user, message):
    mail = mailer.create_message()
    mail.to = user.email
    mail.from_ = 'The store'
    mail.message = message
    mail.send()