# Message Flashing

Good applications and user interfaces are all about feedback. If the user does not get enough feedback they will probably end up hating the application. Flask provides a really simple way to give feedback to a user with the flashing system. The flashing system basically makes it possible to record a message at the end of a request and access it next request and only next request. This is usually combined with a layout template that does this. Note that browsers and sometimes web servers enforce a limit on cookie sizes. This means that flashing messages that are too large for session cookies causes message flashing to fail silently.

## Simple Flashing

So here is a full example:

    from flask import Flask, flash, redirect, render_template, \
         request, url_for

    app = Flask(__name__)
    app.secret_key = b'_5#y2L"F4Q8z\n\xec]/'

    @app.route('/')
    def index():
        return render_template('index.html')

    @app.route('/login', methods=['GET', 'POST'])
    def login():
        error = None
        if request.method == 'POST':
            if request.form['username'] != 'admin' or \
                    request.form['password'] != 'secret':
                error = 'Invalid credentials'
            else:
                flash('You were successfully logged in')
                return redirect(url_for('index'))
        return render_template('login.html', error=error)

And here is the `layout.html` template which does the magic:

    <!doctype html>
    <title>My Application</title>
    {% with messages = get_flashed_messages() %}
      {% if messages %}
        <ul class=flashes>
        {% for message in messages %}
          <li>{{ message }}</li>
        {% endfor %}
        </ul>
      {% endif %}
    {% endwith %}
    {% block body %}{% endblock %}

Here is the `index.html` template which inherits from `layout.html`:

    {% extends "layout.html" %}
    {% block body %}
      <h1>Overview</h1>
      <p>Do you want to <a href="{{ url_for('login') }}">log in?</a>
    {% endblock %}

And here is the `login.html` template which also inherits from `layout.html`:

    {% extends "layout.html" %}
    {% block body %}
      <h1>Login</h1>
      {% if error %}
        <p class=error><strong>Error:</strong> {{ error }}
      {% endif %}
      <form method=post>
        <dl>
          <dt>Username:
          <dd><input type=text name=username value="{{
              request.form.username }}">
          <dt>Password:
          <dd><input type=password name=password>
        </dl>
        <p><input type=submit value=Login>
      </form>
    {% endblock %}

## Flashing With Categories

New in version 0.3.

It is also possible to provide categories when flashing a message. The default category if nothing is provided is `'message'`. Alternative categories can be used to give the user better feedback. For example error messages could be displayed with a red background.

To flash a message with a different category, just use the second argument to the `flash()` function:

    flash('Invalid password provided', 'error')

Inside the template you then have to tell the `get_flashed_messages()` function to also return the categories. The loop looks slightly different in that situation then:

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

This is just one example of how to render these flashed messages. One might also use the category to add a prefix such as `<strong>Error:</strong>` to the message.

## Filtering Flash Messages

New in version 0.9.

Optionally you can pass a list of categories which filters the results of `get_flashed_messages()`. This is useful if you wish to render each category in a separate block.

    {% with errors = get_flashed_messages(category_filter=["error"]) %}
    {% if errors %}
    <div class="alert-message block-message error">
      <a class="close" href="#">×</a>
      <ul>
        {%- for msg in errors %}
        <li>{{ msg }}</li>
        {% endfor -%}
      </ul>
    </div>
    {% endif %}
    {% endwith %}