url_for(‘home’) : Here home is actually the view method name.

use flash() for showing one time messages. First argument is the the actual message that you want to show. flash() accepts a second argument called category. This is commonly used to pass in bootstrap class for flash message. I commonly use ‘success’ or ‘danger’ bootstrap classes.

In your python view:

    if form.validate_on_submit():
        flash(f'Account created for {form.username.data}!', category='success')

1 2	if form.validate_on_submit(): flash(f'Account created for {form.username.data}!', category='success')

Then we display these flash messages in a template file like so:

{% with messages = get_flashed_messages(with_categories=true) %}
          {% if messages %}
            {% for category, message in messages %}
              <div class="alert alert-{{ category }}">{{message }}</div>
            {% endfor %}
          {% endif %}
        {% endwith %}

{% with messages = get_flashed_messages(with_categories=true) %}

{% if messages %}

{% for category, message in messages %}

<div class="alert alert-{{ category }}">{{message }}</div>

{% endfor %}

{% endif %}

{% endwith %}

Here with_categories=true part allows the flashed messages to accept bootstrap classes like ‘success’ that we passed.

username = StringField('Username', validators=[DataRequired(), Length(min=2, max=20)]) here ‘Username’ is automatically designated as label for this form field in the form. So in my html for the corresponding form, I can set both label and actual form like so:

<div class="form-group">
                    {{ form.confirm_password.label(class="form-control-label") }}
                    {{ form.confirm_password(class="form-control form-control-lg") }}
                </div>

{{ form.confirm_password(class="form-control form-control-lg") }}

</div>

How to display validation errors on form fields

We display form fields like so:

<div class="form-group">
                    {{ form.email.label(class="form-control-label") }}
                    {{ form.email(class="form-control form-control-lg") }}
</div>

</div>

Now, if the user entered and email that was not valid, we need to show some feedback message to the user (we call it postback). We do it by using “is-valid” class offered by Bootstrap like so.

<div class="form-group">
    {{ form.email.label(class="form-control-label") }}
    {% if form.email.errors %}
    {{ form.email(class="form-control form-control-lg is-invalid") }}
    <div class="invalid-feedback">
        {% for error in form.email.errors %}
        <span>{{ error }}</span>
        {% endfor %}
    </div>
    {% else %}
        {{ form.email(class="form-control form-control-lg") }}
    {% endif %}
</div>

{% if form.email.errors %}

{{ form.email(class="form-control form-control-lg is-invalid") }}

{% for error in form.email.errors %}

<span>{{ error }}</span>

{% endfor %}

</div>

{% else %}

{% endif %}

</div>

Here, the else block shows the form field when there was no error.

Tek Shinobi

flask

How to display validation errors on form fields