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@app.route('/')
def index():
    if 'username' in session:
        return 'Logged in as %s' % escape(session['username'])
    return 'You are not logged in'

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        if authenticate(str(username), str(password)):
            session['username'] = request.form['username']
            return redirect(url_for('index'))
        else:
            return 'Invalid username/password'
    return render_template('login.html')

@app.route('/logout')
def logout():
    # remove the username from the session if it's there
    session.pop('username', None)
    return redirect(url_for('index'))

{% extends "layout.html" %}
{% block body %}
{% if error %}<p class="error"><strong>Error:</strong> {{ error }}{% endif %}
<form action="{{ url_for('login') }}" method="post">
    <div class="wrap">
        <div class="rpi-logo">
            <img src="static/images/Raspi-PGB001.png">
        </div>
        <input type="text" placeholder="username" name="username" required>
        <div class="bar">
            <i></i>
        </div>
        <input type="password" placeholder="password" name="password" required>
        <button>Sign in</button>
    </div>
</form>
{% endblock %}

<!doctype html>
<title>Web APi</title>
<link rel=stylesheet type=text/css href="{{ url_for('static', filename='style.css') }}">
    <div class=page>
{% for message in get_flashed_messages() %}
    <div class=flash>{{ message }}</div>
{% endfor %}
{% block body %}{% endblock %}
</div>

{% extends "layout.html" %}
{% block body %}
        <div>INDEX.HTML</div>
        {% if error %}<p class="error"><strong>Error:</strong> {{ error }}{% endif %}
        <form action="{{ url_for('preview') }}" method="post">
            <div class="wrap">
                <div class="rpi-logo">
                    <img src="static/images/Raspi-PGB001.png">
                        </div>
                <button>Start Preview</button>
            </div>
        </form>
        {% endblock %}