AttributeError: 'MultiPolygon' object has no attribute 'exterior' in gross_properties

[MestreCarlos]

Hi, I found some little bug when computing gross_properties for a section whose geometry is a MultiPolygon (e.g. disconnected rectangles), an AttributeError is raised because MultiPolygon has no exterior attribute. It’s a minor thing and I don’t think it’s necessary to create a new PR for this.

Location

structuralcodes/sections/_generic.py, method _calculate_gross_section_properties,

Problematic code

if isinstance(polygon, MultiPolygon):
    gp.perimeter = 0.0
    warnings.warn(
        'Perimiter computation for a multi polygon is not defined.'
    )

gp.perimeter = polygon.exterior.length  # ← Always executed; fails with MultiPolygon

The if block sets gp.perimeter = 0.0 for MultiPolygon, but the next line always runs and accesses polygon.exterior, which does not exist on MultiPolygon.

Proposed fix

if isinstance(polygon, MultiPolygon):
    gp.perimeter = 0.0
    warnings.warn(
        'Perimiter computation for a multi polygon is not defined.'
    )
else:
    gp.perimeter = polygon.exterior.length

Code example

from shapely import Polygon

from structuralcodes.geometry import CompoundGeometry, SurfaceGeometry
from structuralcodes.sections import GenericSection
from structuralcodes.materials.basic import ElasticMaterial


mat = ElasticMaterial(E=210000, density=7850)
rect1 = SurfaceGeometry(
    poly=Polygon([(0, 0), (100, 0), (100, 10), (0, 10)]),
    material=mat,
)
rect2 = SurfaceGeometry(
    poly=Polygon([(0, 200), (100, 200), (100, 210), (0, 210)]),
    material=mat,
)

compound = CompoundGeometry([rect1, rect2])
section = GenericSection(compound, integrator="marin")
gp = section.gross_properties

[talledodiego]

Thanks @MestreCarlos for pointing this out! indeed I forgot the else...

Fixed in #332

[talledodiego]

@mortenengen I think this can be closed now that #332 has been merged with dev.