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Empty geometries not all created the same way

See original GitHub issue

Currently there are two ways to create an empty geometry (e.g. Polygon):

from shapely import wkt
from shapely.geometry import mapping, Polygon

# Method 1: using an empty constructor
g1 = Polygon()
print('{}, {}, {}'.format(mapping(g1), g1.geom_type, g1.wkt))
# {'type': 'Polygon', 'coordinates': ()}, GeometryCollection, GEOMETRYCOLLECTION EMPTY

# Method 2: loading empty WKT
g2 = wkt.loads('POLYGON EMPTY')
print('{}, {}, {}'.format(mapping(g2), g2.geom_type, g2.wkt))
# {'type': 'Polygon', 'coordinates': ()}, Polygon, POLYGON EMPTY

(all other geometry types behave the same way).

In both cases, the Python objects know they are Polygons based on the Python object type. However the GEOS objects (referenced by ._geom) are either GeometryCollection or Polygon, based on how the geometries were constructed. This seems inconsistent to me, as I’d expect the two methods to be identical.

Issue Analytics

State:
Created 4 years ago
Reactions:1
Comments:9 (7 by maintainers)

Top GitHub Comments

1reaction

jorisvandenbosschecommented, Aug 12, 2022

Small update on one of the mentioned issues here: the WKB serialization for POINT EMPTY now works and uses the format of PONIT (NaN NaN) (as was already done before by eg PostGIS as well). GEOS itself now does this since version 3.9.0, and with shapely 2.0 we also ensure consistent behaviour for this across all GEOS versions.

1reaction

jorisvandenbosschecommented, Apr 4, 2022

Also, empty geometries have has_z always as False:

Yes, that’s a known limitation. GEOS doesn’t really properly support empty 3D geometries. See also https://github.com/shapely/shapely/issues/1345#issuecomment-1069701607

In the documentation on the main branch we have a note about this in has_z:

https://github.com/shapely/shapely/blob/fc55006ecac8ad7835fe1618d6d391bfe076403c/shapely/predicates.py#L39-L43

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