Serialize python list containing geometry

Question

In simple case if list doesn't contains geometry all works fine:

result = [dict(f.fields, id=f.id, label=f.label) for f in features]
json.dumps(result)

But if result list contains geometry (Shapely) I get error object at 0xb4a87bcc> is not JSON serializable:

import geojson
result = [dict(f.fields, id=f.id, label=f.label, geom=f.geom) for f in features]
geojson.dumps(result)

If I try to serialize not list but one dict - all works fine:

geojson.dumps(result[0])

Is it possible to serialize list of dicts?

Answer 1

Use the mappingfunction of shapely (and the reverse: shape):

from shapely.geometry import Point, mapping, shape
point = Point(3,4)
print mapping(point)
{'type': 'Point', 'coordinates': (3.0, 4.0)}
# and the reverse
print shape(mapping(point))
POINT (3.0000000000000000 4.0000000000000000)

Test:

result = dict(fields="a", id=2, label="label", geom=mapping(point))
geojson.dumps(result)
'{"fields": "a", "geom": {"type": "Point", "coordinates": [3.0, 4.0]}, "id": 2, "label": "label"}'
import json
json.dumps(result)
'{"fields": "a", "geom": {"type": "Point", "coordinates": [3.0, 4.0]}, "id": 2, "label": "label"}'

So, in your case:

result = [dict(f.fields, id=f.id, label=f.label, geom=mapping(f.geom)) for f in features]

Look for examples of the use of Fiona on Gis StackExchange. This module uses the GeoJSON format to read and write shapefiles and other ogr formats..

import fiona
# schema of the shapefile
schema = {'geometry': 'Point', 'properties': {'id':'int:2', 'name':'str'}}
# write the shapefile with the shapely point
with fiona.open('point.shp','w','ESRI Shapefile', schema) as w:
    w.write({'geometry':mapping(point), 'properties':{'id':1, 'name':'a point'}})
# open and read the features of the shapefile
features = fiona.open('point.shp')
features.next()
{'geometry': {'type': 'Point', 'coordinates': (3.0, 4.0)}, 'type': 'Feature', 'id': '0', 'properties': OrderedDict([(u'id', 1), (u'name', u'a point')])}