.. _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. 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 = 'some_secret' @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 :file:`layout.html` template which does the magic: .. sourcecode:: html+jinja My Application {% with messages = get_flashed_messages() %} {% if messages %} {% endif %} {% endwith %} {% block body %}{% endblock %} Here is the :file:`index.html` template which inherits from :file:`layout.html`: .. sourcecode:: html+jinja {% extends "layout.html" %} {% block body %}

Overview

Do you want to log in? {% endblock %} And here is the :file:`login.html` template which also inherits from :file:`layout.html`: .. sourcecode:: html+jinja {% extends "layout.html" %} {% block body %}

Login

{% if error %}

Error: {{ error }} {% endif %}

Username:
Password:

{% 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 %} {% 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 ``Error:`` to the message. Filtering Flash Messages ------------------------ .. versionadded:: 0.9 Optionally you can pass a list of categories which filters the results of :func:`~flask.get_flashed_messages`. This is useful if you wish to render each category in a separate block. .. sourcecode:: html+jinja {% with errors = get_flashed_messages(category_filter=["error"]) %} {% if errors %}
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{% endif %} {% endwith %}