Distance from empty collection is zero

Expected behavior and actual behavior.

Expected behaviour:

The distance from an empty collection should be +inf.

Actual behaviour:

The distance from an empty collection returns zero.

Steps to reproduce the problem.

Run the following script:

from shapely.geometry import Point, LineString

empty_thing = LineString()
assert empty_thing.is_empty
print(empty_thing.distance(Point(1, 1)))  # Returns zero, should probably be +inf

Operating system

Windows 10 64 bit

Shapely version and provenance

Python 3.6 Shapely 1.6b4 installed from PyPI using pip

[mwtoews]

Similar to GEOS, JTS returns 0.0, so perhaps this behavior was drafted a long time ago.

PostGIS returns null for this scenario, since there are not any geometries to measure between. I’d suggest Shapely similarly return None if any of the inputs are empty.

[bixb0012]

SQL Server spatial types return NULL, similar to PostGIS. Esri’s ArcPy generates an error. It seems we have 0, NULL, or error depending on whom you ask. I can see some merit behind arguments for any of them, so clear documentation is probably the most important part of addressing this issue. After all, without a consensus among geospatial products, it is a matter of expectations and not correctness.