Throw error when creating a MultiPoylgon with infinite edges

Expected behavior and actual behavior.

As per the doc:

Figure 7. … on the right, a MultiPolygon that is invalid because its members touch at an infinite number of points (along a line).

Thus shapely should throw an error when creating a MultiPoylgon with infinite edges. (I caught this error when trying to save such a geometry with GeoPandas, and it would always convert the MultiPolygon into a single Polygon.)

Steps to reproduce the problem.

This test should throw an error:

from shapely.geometry import Polygon, MultiPolygon

polygon1 = Polygon(((0, 0), 
                    (0, 1), 
                    (1, 1),
                    (1, 0),
                    (0, 0)))

polygon2 = Polygon(((1, 0), 
                    (1, 1), 
                    (2, 1),
                    (2, 0),
                    (1, 0)))

multipolygon = MultiPolygon([polygon1, polygon2])

Operating system

Linux/Ubuntu

Shapely version and provenance

Shapely 1.6.4.post2 Python 3.5

[sgillies]

@asmith26 the GEOS library used by Shapely has a global context but that context doesn’t give us anything to strictly require valid geometries. We’d need to do this entirely in Python. If you want it to be entirely transparent, so that Polygon(..) raises an exception when invalid, I think the thing to do would be to write a Python context manager that patches all the Shapely geometry constructors when __enter__() is called and then restores them when __exit__() is called. I like writing code like this, but I’m not keen to include it in Shapely. Adding features to the library requires more tests and more documentation and I’m super short on time right now.

[asmith26]

Thank @kannes thanks for this information.

Perhaps there could be a configuration option that one could add to the start of their script to e.g. automatically check validity of all geometries created by the script (and perhaps throw an error instead of just saying “False”). (I appreciate such configurations may add unwanted complications, but I thought I would ask in case something similar already exists or if it inspires other suggestions.)