Geometry saved as MultiPolygon is loaded as Polygon and order changes

Expected behavior and actual behavior.

I save a 3D multipolygon shapefile with row geometry type “MultiPolygon”. When I read this file again I expect that the row geometry type is still “MultiPolygon”. But actually it is downgraded to “Polygon” and also the polygons order in this geometry is changed.

This only happens for certain coordinates (see Run 2 below – only the coordinates changed!).

Steps to reproduce the problem.

import fiona
from pprint import pprint

fn = '/tmp/test_mp.shp'
opts = {
    'driver': 'ESRI Shapefile',
    'schema': {'geometry': '3D MultiPolygon', 'properties': {}}
}

# Run 1 - OK
polygons = [
    [[[0.0, 0.0, 0.0], [1.0, 0.0, 0.0], [0.0, 0.0, 1.0], [0.0, 0.0, 0.0]]],
    [[[0.0, 0.0, 0.0], [2.0, 0.0, 0.0], [0.0, 0.0, 1.0], [0.0, 0.0, 0.0]]]
]

with fiona.open(fn, mode='w', **opts) as c:
    c.write({'geometry': {'type': 'MultiPolygon', 'coordinates': polygons}, 'properties': {}})

with fiona.open(fn, mode='r', **opts) as c:
    feature = next(iter(c))
    pprint(feature['geometry'])
    assert feature['geometry']['type'] == 'MultiPolygon' # ok!

# Run 2 - Failing
polygons = [
        [[[0.0, 0.0, 0.0], [1.0, 0.0, 0.0], [0.0, 0.0, 1.0], [0.0, 0.0, 0.0]]],
        [[[0.0, 0.0, 0.0], [0.0, 1.0, 0.0], [1.0, 0.0, 0.0], [0.0, 0.0, 0.0]]],
    ]

with fiona.open(fn, mode='w', **opts) as c:
    c.write({'geometry': {'type': 'MultiPolygon', 'coordinates': polygons}, 'properties': {}})

with fiona.open(fn, mode='r', **opts) as c:
    feature = next(iter(c))
    pprint(feature['geometry'])
    assert feature['geometry']['type'] == 'MultiPolygon'
    # type is downgraded to 'Polygon' here and polygon order is changed

Operating system

Arch Linux

Fiona and GDAL version and provenance

• Python 3.8
• Fiona==1.8.13.post1 installed from PyPI using pip 20.0.2
• GDAL 3.0.4, released 2020/01/28

[rbuffat]

I’m not familiar with the Shapefile format, respectively the implementation of the gdal Shapefile driver. But after reading https://github.com/OSGeo/gdal/issues/1787#issuecomment-522250212 I suspect it is best to check the validity of the geometry before writing them E.g. by using the shapely library. GeoJSON and GPKG seem to be more tolerant with writing invalid geometries.

It’s not ideal that the Shapefile driver does not raise an error when it can’t handle invalid geometries and does something unexpected. But this is probably something that should be handled within gdal and not fiona.

[sgillies]

I will close this one. In #870 I propose a documentation fix.