A Python function for merging GeoJSON polygons.

polygon_merger.py

import json

def merge_polygons(polygon_one, polygon_two):

    with open("data.json") as input:

        data = json.load(input)

        map_one = None
        map_two = None

        for item in data:

            if item["properties"]["code"] == polygon_one:
                map_one = item
            if item["properties"]["code"] == polygon_two:
                map_two = item

        if map_one and map_two:

            if map_two["geometry"]["type"] == "Polygon":
                map_two["geometry"]["coordinates"] = [map_two["geometry"]["coordinates"]]
                map_two["geometry"]["type"] = "MultiPolygon"

            for coord in map_one["geometry"]["coordinates"]:

                if map_one["geometry"]["type"] == "Polygon":
                    map_two["geometry"]["coordinates"].append([coord])
                elif map_one["geometry"]["type"] == "MultiPolygon":
                    map_two["geometry"]["coordinates"].append(coord)

            for item in data:

                if item["properties"]["code"] == polygon_two:
                    item = map_two
                if item["properties"]["code"] == polygon_one:
                    data.remove(item)

        with open("data.json", "w") as output:

            json.dump(data, output, indent=2, sort_keys=True)