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AJAX with jQuery
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================
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`jQuery`_ is a small JavaScript library commonly used to simplify working
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with the DOM and JavaScript in general. It is the perfect tool to make
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web applications more dynamic by exchanging JSON between server and
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client.
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JSON itself is a very lightweight transport format, very similar to how
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Python primitives (numbers, strings, dicts and lists) look like which is
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widely supported and very easy to parse. It became popular a few years
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ago and quickly replaced XML as transport format in web applications.
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.. _jQuery: http://jquery.com/
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Loading jQuery
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--------------
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In order to use jQuery, you have to download it first and place it in the
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static folder of your application and then ensure it's loaded. Ideally
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you have a layout template that is used for all pages where you just have
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to add a script statement to the bottom of your `<body>` to load jQuery:
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.. sourcecode:: html
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<script type=text/javascript src="{{
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url_for('static', filename='jquery.js') }}"></script>
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Another method is using Google's `AJAX Libraries API
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<https://developers.google.com/speed/libraries/devguide>`_ to load jQuery:
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.. sourcecode:: html
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<script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
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<script>window.jQuery || document.write('<script src="{{
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url_for('static', filename='jquery.js') }}">\x3C/script>')</script>
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In this case you have to put jQuery into your static folder as a fallback, but it will
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first try to load it directly from Google. This has the advantage that your
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website will probably load faster for users if they went to at least one
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other website before using the same jQuery version from Google because it
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will already be in the browser cache.
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Where is My Site?
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-----------------
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Do you know where your application is? If you are developing the answer
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is quite simple: it's on localhost port something and directly on the root
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of that server. But what if you later decide to move your application to
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a different location? For example to ``http://example.com/myapp``? On
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the server side this never was a problem because we were using the handy
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:func:`~flask.url_for` function that could answer that question for
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us, but if we are using jQuery we should not hardcode the path to
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the application but make that dynamic, so how can we do that?
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A simple method would be to add a script tag to our page that sets a
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global variable to the prefix to the root of the application. Something
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like this:
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.. sourcecode:: html+jinja
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<script type=text/javascript>
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$SCRIPT_ROOT = {{ request.script_root|tojson|safe }};
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</script>
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The ``|safe`` is necessary in Flask before 0.10 so that Jinja does not
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escape the JSON encoded string with HTML rules. Usually this would be
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necessary, but we are inside a `script` block here where different rules
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apply.
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.. admonition:: Information for Pros
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In HTML the `script` tag is declared `CDATA` which means that entities
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will not be parsed. Everything until ``</script>`` is handled as script.
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This also means that there must never be any ``</`` between the script
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tags. ``|tojson`` is kind enough to do the right thing here and
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escape slashes for you (``{{ "</script>"|tojson|safe }}`` is rendered as
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``"<\/script>"``).
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In Flask 0.10 it goes a step further and escapes all HTML tags with
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unicode escapes. This makes it possible for Flask to automatically
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mark the result as HTML safe.
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JSON View Functions
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-------------------
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Now let's create a server side function that accepts two URL arguments of
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numbers which should be added together and then sent back to the
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application in a JSON object. This is a really ridiculous example and is
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something you usually would do on the client side alone, but a simple
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example that shows how you would use jQuery and Flask nonetheless::
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from flask import Flask, jsonify, render_template, request
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app = Flask(__name__)
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@app.route('/_add_numbers')
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def add_numbers():
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a = request.args.get('a', 0, type=int)
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b = request.args.get('b', 0, type=int)
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return jsonify(result=a + b)
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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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As you can see I also added an `index` method here that renders a
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template. This template will load jQuery as above and have a little form
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we can add two numbers and a link to trigger the function on the server
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side.
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Note that we are using the :meth:`~werkzeug.datastructures.MultiDict.get` method here
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which will never fail. If the key is missing a default value (here ``0``)
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is returned. Furthermore it can convert values to a specific type (like
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in our case `int`). This is especially handy for code that is
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triggered by a script (APIs, JavaScript etc.) because you don't need
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special error reporting in that case.
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The HTML
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--------
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Your index.html template either has to extend a `layout.html` template with
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jQuery loaded and the `$SCRIPT_ROOT` variable set, or do that on the top.
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Here's the HTML code needed for our little application (`index.html`).
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Notice that we also drop the script directly into the HTML here. It is
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usually a better idea to have that in a separate script file:
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.. sourcecode:: html
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<script type=text/javascript>
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$(function() {
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$('a#calculate').bind('click', function() {
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$.getJSON($SCRIPT_ROOT + '/_add_numbers', {
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a: $('input[name="a"]').val(),
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b: $('input[name="b"]').val()
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}, function(data) {
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$("#result").text(data.result);
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});
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return false;
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});
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});
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</script>
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<h1>jQuery Example</h1>
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<p><input type=text size=5 name=a> +
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<input type=text size=5 name=b> =
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<span id=result>?</span>
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<p><a href=# id=calculate>calculate server side</a>
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I won't go into detail here about how jQuery works, just a very quick
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explanation of the little bit of code above:
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1. ``$(function() { ... })`` specifies code that should run once the
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browser is done loading the basic parts of the page.
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2. ``$('selector')`` selects an element and lets you operate on it.
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3. ``element.bind('event', func)`` specifies a function that should run
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when the user clicked on the element. If that function returns
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`false`, the default behavior will not kick in (in this case, navigate
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to the `#` URL).
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4. ``$.getJSON(url, data, func)`` sends a `GET` request to `url` and will
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send the contents of the `data` object as query parameters. Once the
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data arrived, it will call the given function with the return value as
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argument. Note that we can use the `$SCRIPT_ROOT` variable here that
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we set earlier.
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If you don't get the whole picture, download the `sourcecode
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for this example
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<https://github.com/mitsuhiko/flask/tree/master/examples/jqueryexample>`_
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from github.
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