{% if form.errors | length %}
{{ form.username(class_="form-control is-invalid", placeholder="username") }}
{% else %}
{{ form.username(class_="form-control", placeholder="username") }}
{% endif %}
{% for field, errors in form.errors.items() %}
{{ ', '.join(errors) }}
{% endfor %}
{{ form.password(class_="form-control", placeholder="password") }}