sending data as JSON object from Python to Javascript with Jinja [duplicate]

Question

I'm trying to send a lat/long point as a JSON object from Python to a javascript. I'm using Flask so the following is Jinja templating..

Python:

@app.route('/')
def homepage():
    lat_lng = (39.7392,-104.9847) 
    return render_template("index_v2.html", lat_lng=json.dumps(lat_lng))

html with js:

<script type='text/javascript'>
    var map;
    function initialize() {
      // Create the map.
      var lat_lng = eval('({{ lat_lng }})')
      map = new google.maps.Map(document.getElementById('map-canvas'), {
        zoom: 8,
        center: new google.maps.LatLng(lat_lng)
      });

    }

    google.maps.event.addDomListener(window, 'load', initialize);
  </script>

I'm using the eval because the standard Jinja notation of variable = {{ data }} isn't working and I found some advice that eval was necessary. Any advice?

Answer 1

The Flask Jinja2 documentation covers this pretty well. The first example under the "Standard Filters" section shows exactly how to embed a JSON object from python into a Javascript script:

<script type=text/javascript>
    doSomethingWith({{ user.username|tojson|safe }});
</script>

So in this case:

var lat_lng = {{ lat_lng|tojson|safe }};

tojson calls dumps on the data, so you should pass the data directly to the template rather than calling dumps on it, otherwise you double-serialize the data and end up with a JSON string.