Convert Polygon to Multipolygon with GeoPandas

Question

I'm trying to create a vectors grid from a single geometry, from PostGIS's table to another PostGIS's table; both tables are in the same DB. The input geometry is MultiPolygon and I've previously created with SQL the output table:

CREATE TABLE IF NOT EXISTS comuni_campania_copy (
    id SERIAL PRIMARY KEY,
    comune VARCHAR(255) NOT NULL,
    comune_id INTEGER NOT NULL
);
SELECT AddGeometryColumn('comuni_campania_copy', 'geom', 32633, 'MULTIPOLYGON', 2);

I'm be able to create with GeoPandas a vectors grid from a shapefile and save it as another shapefile. I've modified my script to use this functionality with PostGIS.

from shapely.geometry import MultiPolygon
from sqlalchemy import *
import geopandas as gpd
import numpy as np
import psycopg2

# Connection data
con_params = create_engine('postgresql://postgres:1234@localhost:5432/postgres', echo=False)

# SQL parameters
sql_params_in = "SELECT id as comune_id, geom, comune FROM comuni_campania WHERE id = 3"


def vector_grid_from_db(sql_params: str, con_params: psycopg2.extensions.connection, dx: int, dy: int) -> None:
    # Read vector
    vector_in = gpd.GeoDataFrame.from_postgis(
        sql=sql_params,
        con=con_params
    )

    # Read bounding box
    bbox = vector_in.bounds

    # Generate columns and rows
    cols = list(np.arange(bbox.minx[0], bbox.maxx[0], dx))
    rows = list(np.arange(bbox.miny[0] + dy, bbox.maxy[0] + dy, dy))

    # Create vector grid
    polygons = []
    for x in cols:
        for y in rows:
            polygons.append(MultiPolygon([(x, y), (x + dx, y), (x + dx, y - dy), (x, y - dy)]))

        grid = gpd.GeoDataFrame(geometry=polygons, crs=vector_in.crs)

    epsq_code = int(str(vector_in.crs)[5:])

    grid.to_postgis(
        'comuni_campania_copy',
        con_params,
        if_exists="append",
        index=False,
        dtype={
            "geometry": Geometry(
                "MULTIPOLYGON",
                srid=epsq_code
            )
        }
    )

vector_grid_from_db(sql_params_in, con_params, 300, 300)

Using my script I see this error:

Traceback (most recent call last): File "create_grid_to_db.py", line 71, in vector_grid_from_db(sql_params_in, con_params, 300, 300) File "create_grid_to_db.py", line 45, in vector_grid_from_db polygons.append(MultiPolygon([(x, y), (x + dx, y), (x + dx, y - dy), (x, y - dy)])) File "/home/maxdragonheart/DEV_FOLDER/MIO/GISPython/devenv/lib/python3.8/site-packages/shapely/geometry/multipolygon.py", line 62, in init self._geom, self._ndim = geos_multipolygon_from_polygons(polygons) File "/home/maxdragonheart/DEV_FOLDER/MIO/GISPython/devenv/lib/python3.8/site-packages/shapely/geometry/multipolygon.py", line 181, in geos_multipolygon_from_polygons N = len(exemplar[0][0]) IndexError: invalid index to scalar variable.

Answer 1

A MultiPolygon is a list of Polygons so use:

polygons = []
for x in cols:
    for y in rows:
        polygons.append(MultiPolygon([Polygon([(x, y), (x + dx, y), (x + dx, y - dy), (x, y - dy)])]))