BUG: The union_all feature generates an unexpected result

[thomas-leduc]

Please note

Expected behavior and actual behavior.

The union_all feature produces a MultiLineString with not 2 but 3 LineStrings.

Steps to reproduce the problem.

from shapely import LineString, union_all

geoms = [
	LineString([ (355041.15, 6688781.25, 0), (355040.9629213488, 6688781.437078651, 9.7) ]),
	LineString([ (355041.1500000001, 6688781.25, 0), (354841.1500000001, 6688781.25, 0) ])
]

result = union_all(geoms)

assert len(result.geoms) == len(geoms), "Bug"

Operating system

Ubuntu 22.04.4 LTS (x86_64)

Shapely version and provenance

2.0.3

[theroggy]

I think your code sample isn't ideal, as the lines don't have a common point, but you are right anyway that union_all doesn't union/merge lines.

Not sure why this is split up (I asked if it is intentional here libgeos/geos#1063), but to union/merge lines you need to use shapely.line_merge:

from shapely import LineString, MultiLineString, line_merge, union_all

geoms = [
	LineString([ (355041.15, 6688781.25, 0), (355040.9629213488, 6688781.437078651, 9.7) ]),
	LineString([ (355041.15, 6688781.25, 0), (354841.1500000001, 6688781.25, 0) ])
]

line_merge_result = line_merge(MultiLineString(geoms))
print(line_merge_result)
# LINESTRING Z (354841.1500000001 6688781.25 0, 355041.15 6688781.25 0, 355040.9629213488 6688781.437078651 9.7)

union_all_result = union_all(geoms)
print(union_all_result)
# MULTILINESTRING Z ((355041.15 6688781.25 0, 355040.9629213488 6688781.437078651 9.7), (355041.15 6688781.25 0, 354841.1500000001 6688781.25 0))