tp.Topology.to_json(pretty=True) doesn't handle None correctly. (Doens't convert None to null)

[yizongk]

Hi,

A Geopanda dataframe has None in one of its column, and then is converted to topology . When you want to export it with Topology.to_json(pretty=True), with pretty, you will notice that it doesn't convert any Nones to nulls.

Check this example (Notice "end_date": None)

import geopandas as gpd
import topojson as tp

gdf = gpd.GeoDataFrame.from_features({
    "type": "FeatureCollection",
    "features": [
        {
            "type": "Feature",
            "properties": {
                "end_date": None,
                "pjct_name": "Central Park Drives Closures",
                "shape_leng": "24290.821126",
                "sip_year": "2015"
            },
            "geometry": {
                "type": "MultiLineString",
                "coordinates": [
                    [
                        [
                            -73.95894908875633,
                            40.78413830809674
                        ],
                        [
                            -73.95888401858302,
                            40.78422633413453
                        ]
                    ],
                ]
            }
        }
    ]
})

print(tp.Topology(gdf, prequantize=False).to_json(pretty=True))

You will see this output:

{
    "type": "Topology",
    "objects": {
        "data": {
            "geometries": [
                {
                    "properties": {
                        "end_date": None,
                        "pjct_name": "Central Park Drives Closures",
                        "shape_leng": "24290.821126",
                        "sip_year": "2015"
                    },
                    "type": "MultiLineString",
                    "arcs": [[0]]
                }
            ],
            "type": "GeometryCollection"
        }
    },
    "bbox": [-73.95894908875633, 40.78413830809674, -73.95888401858302, 40.78422633413453],
    "arcs": [
        [[-73.95894908875633, 40.78413830809674], [-73.95888401858302, 40.78422633413453]]
    ]
}

Under "properties" -> "end_date", you can see it's set to None, which is not correct JSON syntax. It should be null.

to_json() without pretty=True will output the correct format with null

[yizongk]

I think the issues stems from utils.py

topojson/topojson/utils.py

Lines 308 to 315 in 1c84fb4

	def basictype2str(obj):
	if isinstance(obj, str):
	strobj = '"' + str(obj) + '"'
	elif isinstance(obj, bool):
	strobj = {True: "true", False: "false"}[obj]
	else:
	strobj = str(obj)
	return strobj

I think a fix would be to put a check to see if obj is None, and then convert it to 'null', like:

# convert None to 'null' since None is not a valid syntax in JSON
elif obj is None:
    strobj = 'null'