confirm.html

{% extends "base.html" %}

{% block content %}
<form method="POST" action="{{ dest }}">
  <input type="hidden" name="confirm" value="no" />
  <input type="submit" value="No" />
</form>
<form method="POST" action="{{ dest }}">
  <input type="hidden" name="confirm" value="yes" />
  <input type="submit" value="Yes" />
</form>
{% endblock %}

confirm.py

def confirm(f):
    @wraps(f)
    def decorated(*args, **kwargs):
        if 'confirm' not in request.form:
            dest = request.url
            context = {'dest': dest}
            return render_template('confirm.html', **context)
        if request.form['confirm'] != 'yes':
            return redirect(url_for('list_galleries'))
        return f(*args, **kwargs)
    return decorated