How to Convert db.ListProperty(GeoPt) to GeoJSON Polygon

Question

My Python Google App Engine (no tag?) app has a datastore model of an Area of Interest, simplified as

import geojson
class AreaOfInterest(db.Model):
    name = db.StringProperty(required=True)
    #Geometry of area boundary
    coordinates = db.ListProperty(db.GeoPt, default=None) #convex polygon without holes

I wish to export this as GeoJSON, but am unsure of a (preferably Pythonic) way to convert the List of GeoPt to a geojson.Polygon object. Although I can convert to an array of Points with

coords = []
for c in self.coordinates:
    p = geojson.Point((c.lat, c.lon))
    coords.append(p)

I wish to embedd a Polygon into a GeoJson FeatureCollection thus:

def geojsonArea(self):
    polygon = geojson.Polygon((self.coordinates)) 
    #ValueError: Expect __geo_interface__' or a mapping, got datastore_types.GeoPt
    geojson_obj =   { 
        "type": "FeatureCollection",
        "area_name" :self.name,
        "features": [
          { "type": "Feature",
             "geometry": {
               "type": "Polygon",
               "coordinates": polygon
             },
             "properties": {
                "name": "boundary"
           }
        }
        ]
    }
    return geojson.dumps(geojson_obj)

But the call to geojson.Polygon expects something like Polygon([[(2.38, 57.322), (23.194, -20.28), (-120.43, 19.15), (2.38, 57.322)]])

What is the cleanest way to make this conversion? Is it possible without looping through the list while building a string to pass to Polygon()?

Answer 1

This might not be the most Pythonic solution but it works. The resulting string complies with http://geojsonlint.com/.

def geojsonArea(self):
    coords = []
    for c in self.coordinates:
        p = {'lat': c.lat, 'lng': c.lon}
        coords.append(p)

    geojson_obj =   { 
        "type": "FeatureCollection",
        "area_name" :self.name,
        "features": [
          { "type": "Feature",
             "geometry": {
                 "type": "Polygon", 
                 "coordinates" : coords
            },
             "properties": {
                "name": "boundary"
           }
        }
        ]
    }
    return geojson.dumps(geojson_obj)

Converting a db.GeoPt to a Point has a different syntax to Polygon.

map_center = db.GeoPtProperty()     

def geojsonArea(self):
    center = []
    center.append(geojson.Point({'lat': self.map_center.lat, 'lng': self.map_center.lon}))
    #... 
    { 
      "type": "ViewPort",
      "geometry": center, 
    }