{% if bad_password %}
{{ form.username(class="form-control is-invalid", placeholder="username") }}
{% else %}
{{ form.username(class="form-control", placeholder="username") }}
{% endif %}
{% if bad_password %}
{{ form.password(class="form-control is-invalid", placeholder="password") }}
{% else %}
{{ form.password(class="form-control", placeholder="password") }}
{% endif %}