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+.. _message-flashing-pattern:
+
+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.
+
+Simple Flashing
+---------------
+
+So here is a full example::
+
+    from flask import flash, redirect, url_for, render_template
+
+    @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 the ``layout.html`` template which does the magic:
+
+.. sourcecode:: html+jinja
+
+   <!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 %}
+
+And here the index.html template:
+
+.. sourcecode:: html+jinja
+
+   {% extends "layout.html" %}
+   {% block body %}
+     <h1>Overview</h1>
+     <p>Do you want to <a href="{{ url_for('login') }}">log in?</a>
+   {% endblock %}
+
+And of course the login template:
+
+.. sourcecode:: html+jinja
+
+   {% extends "layout.html" %}
+   {% block body %}
+     <h1>Login</h1>
+     {% if error %}
+       <p class=error><strong>Error:</strong> {{ error }}
+     {% endif %}
+     <form action="" 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
+------------------------
+
+.. versionadded:: 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 :func:`~flask.flash` function::
+
+    flash(u'Invalid password provided', 'error')
+
+Inside the template you then have to tell the
+:func:`~flask.get_flashed_messages` function to also return the
+categories.  The loop looks slightly different in that situation then:
+
+.. sourcecode:: html+jinja
+
+   {% 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.