How to flash success and danger with different messages in flask.

You can use the following pattern to specify the category parameter of the flash function.

:param category: the category for the message. The following values are recommended: 'message' for any kind of message, 'error' for errors, 'info' for information messages and 'warning' for warnings. However any kind of string can be used as category.

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

By putting category in the class attribute, you can associate special colors with some css rules like :

.alert.success {
  background-color: green;
}

.alert.error {
  background-color: red;
}

---

flash('Message sent successfully', 'success')
flash("Please enter a valid email address", 'error')

These calls will generate:

<div class="alert success">  Message sent successfully </div>
<div class="alert error">  Please enter a valid email address </div>

---

Official documentation : http://flask.pocoo.org/docs/1.0/patterns/flashing/#flashing-with-categories

With flask 1.0.2 this similar but slightly different approach worked for me - slightly simpler I think:

flash('This is an error message in red', 'danger')

flash('This is an informational message in blue', 'info')

In Flask HTML template:

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

This way I didn't need to define any CSS and just used what was already in the default Flask distribution.