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.. _message-flashing-pattern:
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Message Flashing
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================
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Good applications and user interfaces are all about feedback. If the user
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does not get enough feedback he will probably end up hating the
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application. Flask provides a really simple way to give feedback to a
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user with the flashing system. The flashing system basically makes it
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possible to record a message at the end of a request and access it next
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request and only next request. This is usually combined with a layout
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template that does this.
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So here a full example::
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from flask import flash, redirect, url_for, render_template
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@app.route('/')
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def index():
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return render_template('index.html')
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@app.route('/login', methods=['GET', 'POST'])
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def login():
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error = None
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if request.method == 'POST':
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if request.form['username'] != 'admin' or \
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request.form['password'] != 'secret':
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error = 'Invalid credentials'
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else:
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flash('You were sucessfully logged in')
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return redirect(url_for('index'))
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return render_template('login.html', error=error)
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And here the ``layout.html`` template which does the magic:
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.. sourcecode:: html+jinja
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<!doctype html>
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<title>My Application</title>
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{% with messages = get_flashed_messages() %}
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{% if messages %}
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<ul class=flashes>
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{% for message in messages %}
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<li>{{ message }}</li>
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{% endfor %}
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</ul>
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{% endif %}
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{% endwith %}
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{% block body %}{% endblock %}
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And here the index.html template:
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.. sourcecode:: html+jinja
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{% extends "layout.html" %}
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{% block body %}
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<h1>Overview</h1>
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<p>Do you want to <a href="{{ url_for('login') }}">log in?</a>
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{% endblock %}
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And of course the login template:
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.. sourcecode:: html+jinja
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{% extends "layout.html" %}
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{% block body %}
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<h1>Login</h1>
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{% if error %}
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<p class=error><strong>Error:</strong> {{ error }}
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{% endif %}
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<form action="" method=post>
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<dl>
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<dt>Username:
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<dd><input type=text name=username value="{{
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request.form.username }}">
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<dt>Password:
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<dd><input type=password name=password>
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</dl>
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<p><input type=submit value=Login>
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</form>
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{% endblock %}
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