How to return JSON data from Flask without ajax

Question

Suppose I want to render a page (not just JSON) using Flask with some specific data that I fetch from the database. For example

display_data.html includes:

<script src='display_data.js'></script>
...
<h1>Data display page!</h1>
<div id="chartContainer"></div>

display_data.js:

$(function() {
     draw_chart($("#chartContainer"), json_data); 
     //draw_chart is defined elsewhere and json_data is what I want to pass in
});

Python:

@app.route('/<data_id>')
def get_display_data_page(data_id):
  data = get_data_by_id(data_id)
  return render_template('display_data.html', data = data)

I think that if I want to just "render template", I'd have to include elsewhere in display_data.html the following:

<script>window.json_data = {{ data | tojson | safe}}</script>

This pattern smells bad: I'm leaving an object on the global namespace (so that my JS file can access it), displaying the data as plain text, and rendering a string in that is parsed into JSON so the JS can use it. Looks bad but this does work.

Two other options:

Return the data with AJAX. Given the title of this post I'm specifically trying to avoid ajax. The reason for this is mainly that I'm building a mobile site and want to reduce the number of pings back to the server. I'm also thinking (perhaps more metaphysically) about encapsulating the page: once you have it, you have all of it.
Render my JS file via Flask and Jinja. This seems like a bummer because I'd have to then write a route down and render the JS based on the same logic that I have in the get_display_data_page: looking up the data by its id, etc. Code duplication and dynamic JS sound like big no-no's to me.

Is there a known pattern to doing this well?

Sean Vieira · Accepted Answer · 2014-02-14 22:00:03Z

There's no need to leave data in the global scope if you don't want to. In your template you can do something like this:

<script>
function registerTask(f, args) {
  $(function() {
    f.call(this, args);
  });
}

{% for name, args in js_tasks %}
  registerTask({{name}}, {{ args | tojson | safe }});
{% endfor %}
</script>

Then, in your JS file, redefine draw_chart to just take the data (or have a wrapper around it that you use as your task registry name):

function draw_chart_task(data) {
  draw_chart($('#chartContainer'), data);
}

Finally, in your controller, simply provide the data and the task name as a tuple:

return render_template('display_data.html', js_tasks=[('draw_chart_task', data)])

This ensures that your JavaScript is not just plucking its dependencies out of the global scope, and you are not making extra network calls.

The data is visible in the raw text output of the page, but it is visible if you make an AJAX call too, you just have to look in a different panel of your browser's developer's tools to see it.

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