How to return WKT of entire polygon with shapely?

Question

Im learning to use shapely and have two shapefiles--forest, and basins. The two shapefiles clearly intersect (basins is completely within forest). But the way I've gone about it only gets the feature at index 0 of each shapefile, which don't intersect. I know where this is taking place in the script, but how do I get the whole polygon shape as wkt instead of just one record? Do I build a list of the wkt geometry one record at a time?

# Basins shapefile to wkt
basins = ogr.Open("data/Basins.shp")
basins_layer = basins.GetLayer(0)
basins_feature = basins_layer.GetFeature(0) # This is the line
# I'm thinking there should probably be a loop here to get the geometry of the whole shapefile
basins_geometry = basins_feature.GetGeometryRef()
basins_wkt = shapely.wkt.loads(basins_geometry.ExportToWkt())

How do I loop through the whole shapefile and build the outer polygon as wkt or is there another function to get the outer boundary of the shapefile? I guess this is called the outer ring. I want wkt of the whole shapefile exterior, rather than the individual polygons that make it up.

Answer 1

You can just use .wkt as wkt_polygon = ShapelyPolygon(..).wkt

Answer 2

To get wkt outer ring of a polygon with shapely, you can use next code with your particular paths:

import fiona
from shapely.geometry import shape, LineString

path = '/home/zeito/pyqgis_data/polygon1.shp' #polygon with only one feature

c = fiona.open(path)

collection = [ shape(item['geometry']) for item in c ]

rings = [ LineString(pol.coords).wkt for pol in collection ]

print rings[0] #0 because polygon has only one feature (one outer ring)

After running above code at Python Console, for my particular path, I got:

LINESTRING (389535.8208391897 4448641.082016046, 397951.9055779715 4459595.351041127, 418925.3231016023 4456522.812168239, 425070.4008473795 4441160.117803795, 412513.0680625305 4427801.253139063, 392341.1824187837 4435148.628704665, 389535.8208391897 4448641.082016046)

By using QuickWKT plugin of QGIS, it can be observed that it works as expected:

Editing Note:

If you have issues to install fiona, an alternative code, by using ogr python module, is the following:

from osgeo import ogr
from shapely.wkt import loads
from shapely.geometry import LineString

path = '/home/zeito/pyqgis_data/polygon1.shp'

basins = ogr.Open(path)

layer = basins.GetLayer()

geoms = []

for feature in layer:
    geom = feature.GetGeometryRef()
    geoms.append(geom.ExportToWkt())

pol = loads(geoms[0])

print LineString(pol.exterior.coords).wkt

It produces same result that first code.