How to detect inner polygons from a multipolygon shapely object?

Question

I would like to detect inner polygons from a multipolygon shapely object. Great lakes, Black Sea and Caspian sea should be inner polygons and not be filled.

How to do this properly with shapefile?

Please find the script bellow for investigating.

import matplotlib.pyplot as plt
import cartopy.crs as ccrs
from shapely import geometry
import random
import pickle

! wget -nc https://thredds-su.ipsl.fr/thredds/fileServer/ipsl_thredds/brocksce/tmp/polys.pickle

with open('./polys.pickle', "rb") as poly_file:
    polygons = pickle.load(poly_file)

fig = plt.figure(figsize=(10,5))
ax = fig.add_subplot(1, 1, 1, projection=ccrs.Robinson(10))

transform = ccrs.Geodetic()

for polygon in polygons.geoms:
    random_color = "#"+''.join([random.choice('0123456789ABCDEF') for i in range(6)])
    x = polygon.exterior.coords.xy[0]
    y = polygon.exterior.coords.xy[1]
    ax.fill(x, y, transform=transform, color=random_color, lw=0.5, edgecolor="black")

ax.set_global()
ax.gridlines()
plt.show()

Answer 1

Here is a solution using patches because you can handle polygons with holes. Note that exterior ring will be oriented counter-clockwise and the interior rings (holes) will be oriented clockwise.

import matplotlib.pyplot as plt
from matplotlib.path import Path
from matplotlib.patches import PathPatch
import shapely
from shapely import geometry
import cartopy.crs as ccrs
import random
import pickle

#-----------------------------------------
! wget -nc https://thredds-su.ipsl.fr/thredds/fileServer/ipsl_thredds/brocksce/tmp/polys.pickle

with open('./polys.pickle', "rb") as poly_file:
    polygons = pickle.load(poly_file)

#-----------------------------------------
def polygon2patch(poly, **kwargs):
    path = Path.make_compound_path(
           Path(poly.exterior.coords),
           *[Path(ring.coords) for ring in poly.interiors])
    patch = PathPatch(path, **kwargs)
    return patch

#-----------------------------------------
fig = plt.figure(figsize=(10,5))

map_proj = ccrs.Robinson(-10)
#map_proj = ccrs.Orthographic(-10, -60)
ax = fig.add_subplot(1, 1, 1, projection=map_proj)

transform = ccrs.Geodetic()

holesNumber = []
for n,polygonA in enumerate(polygons.geoms):
    holes = []
    for m,polygonB in enumerate(polygons.geoms):
        if (m == n): continue     
        if polygonA.contains(polygonB):
            holes.append(polygonB.exterior.coords)
            holesNumber.append(m)
    if n in holesNumber: continue  # n is a hole
    polygonNew = geometry.Polygon(polygonA.exterior.coords, holes=holes)
    polygonNew = shapely.geometry.polygon.orient(polygonNew)   # Orient to be oriented counter-clockwise
    random_color = "#"+''.join([random.choice('0123456789ABCDEF') for i in range(6)])
    patch = polygon2patch(polygonNew, transform=transform, facecolor=random_color, lw=0.5, edgecolor="black")
    ax.add_patch(patch)

ax.set_global()
ax.gridlines()
plt.show()

Great lakes, Black Sea and Caspian sea are now not filled.

But notice the Antartica is not drawn correctly. Do not understand why?

More visible the problem of the uncorrect filling with an Orthographic projection. See violet straight filling over Africa. Any help on this point as well would be welcomed?

Have posted an issue about this: https://github.com/SciTools/cartopy/issues/2111