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150 lines
4.5 KiB
150 lines
4.5 KiB
.. _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 they 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. Note that browsers and sometimes web servers enforce |
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a limit on cookie sizes. This means that flashing messages that are too |
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large for session cookies causes message flashing to fail silently. |
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Simple Flashing |
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--------------- |
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So here is a full example:: |
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from flask import Flask, flash, redirect, render_template, \ |
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request, url_for |
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app = Flask(__name__) |
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app.secret_key = b'_5#y2L"F4Q8z\n\xec]/' |
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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 successfully 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 is the :file:`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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Here is the :file:`index.html` template which inherits from :file:`layout.html`: |
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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 here is the :file:`login.html` template which also inherits from |
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:file:`layout.html`: |
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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 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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Flashing With Categories |
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------------------------ |
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.. versionadded:: 0.3 |
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It is also possible to provide categories when flashing a message. The |
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default category if nothing is provided is ``'message'``. Alternative |
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categories can be used to give the user better feedback. For example |
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error messages could be displayed with a red background. |
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To flash a message with a different category, just use the second argument |
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to the :func:`~flask.flash` function:: |
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flash(u'Invalid password provided', 'error') |
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Inside the template you then have to tell the |
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:func:`~flask.get_flashed_messages` function to also return the |
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categories. The loop looks slightly different in that situation then: |
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.. sourcecode:: html+jinja |
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{% with messages = get_flashed_messages(with_categories=true) %} |
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{% if messages %} |
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<ul class=flashes> |
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{% for category, message in messages %} |
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<li class="{{ category }}">{{ 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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This is just one example of how to render these flashed messages. One |
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might also use the category to add a prefix such as |
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``<strong>Error:</strong>`` to the message. |
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Filtering Flash Messages |
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------------------------ |
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.. versionadded:: 0.9 |
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Optionally you can pass a list of categories which filters the results of |
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:func:`~flask.get_flashed_messages`. This is useful if you wish to |
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render each category in a separate block. |
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.. sourcecode:: html+jinja |
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{% with errors = get_flashed_messages(category_filter=["error"]) %} |
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{% if errors %} |
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<div class="alert-message block-message error"> |
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<a class="close" href="#">×</a> |
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<ul> |
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{%- for msg in errors %} |
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<li>{{ msg }}</li> |
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{% endfor -%} |
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</ul> |
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</div> |
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{% endif %} |
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{% endwith %}
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