How to return GeoJSON with Flask to Openlayers

Question

I have a simple flask function that renders a template with a valid GeoJSON string:

@app.route('/json', methods=['POST'])
def json():    
    polygon = Polygon([[[0,1],[1,0],[0,0],[0,1]]])
    return render_template('json.html',string=polygon)

In my json.html file, I am attempting to render this GeoJSON with OpenLayers:

 function init(){
            map = new OpenLayers.Map( 'map' );
            layer = new OpenLayers.Layer.WMS( "OpenLayers WMS", 
                    "http://vmap0.tiles.osgeo.org/wms/vmap0",
                    {layers: 'basic'} );
            map.addLayer(layer);
            map.setCenter(new OpenLayers.LonLat(lon, lat), zoom);
            var fc = {{string}};        //Here is the JSON string
            var geojson_format = new OpenLayers.Format.GeoJSON();
            var vector_layer = new OpenLayers.Layer.Vector(); 
            map.addLayer(vector_layer);
            vector_layer.addFeatures(geojson_format.read(fc));

But this fails and the " characters become '. I have tried string formatting as seen in this question, but it didn't work.

EDIT:

I did forget to dump my json to an actual string, I'm using the geojson library so adding the function

dumps(polygon)

takes care of that, however I still can't parse the GeoJSON in OpenLayers, even though it is a valid string according to geojsonlint.com

This is the Javascript code to create a variable from the string sent from flask:

var geoJson = '{{string}}';

And here's what it looks like in the source page:

'{&#34;type&#34;: &#34;Polygon&#34;, &#34;coordinates&#34;: [[[22.739485934746977, 39.26596659794341], [22.73902517923571, 39.266115931275074], [22.738329551588276, 39.26493626464484], [22.738796023230854, 39.26477459496181], [22.739485934746977, 39.26596659794341]]]}';

I am still having a problem rendering the quote characters.

tbicr · Accepted Answer · 2014-06-17T16:40:11.517

1

Look like you use shapely which has http://toblerity.org/shapely/shapely.geometry.html#shapely.geometry.mapping method to create GeoJSON-like object.

To render json use tojson filter which safe (see safe filter) for latest flask versions, because jinja2 in flask by default escape all dangerous symbols to protect XSS.

edited Jun 17 '14 at 16:40

answered Jun 17 '14 at 12:52

tbicr

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That was part of the problem, but still not there, see my edits. – camdenl Jun 17 '14 at 15:44
All work right, dangerous symbols escaped by default with `jinja2` to protect XSS. To insert safe markup you should mark template variable as safe with `safe` filter (see `{{ string|safe }}`), but for json better use special filter `tojson` which also safe (for latest flask versions at least) and equal to `{{ flask.json.dupms(polygon)|safe }}` (see `{{ string|tojson }}`). – tbicr Jun 17 '14 at 16:24
This is the correct answer, mind adding it to your answer text? – camdenl Jun 17 '14 at 16:33

How to return GeoJSON with Flask to Openlayers

1 Answers1