gpd.to_file could support GeoDataFrames with multiple geometry types

Context

Since Fiona 446 resolution (see PR 539), fiona.open allows to provide a schema with a list of geometry types, or even any types (using Unknown keyword)

Current behaviour

Currently, gpd.to_file fails for the following cases (using GeoJSON OGR driver):

• a gdf containing a shape type and the corresponding Multi shape type (e.g. Polygon and MultiPolygon): the inferred schema contains only the simple shape type (e.g. Polygon), but then fiona raises the error below:

     File "fiona/ogrext.pyx", line 1182, in fiona.ogrext.WritingSession.writerecs
   fiona.errors.GeometryTypeValidationError: Record's geometry type does not match collection schema's geometry type: 'MultiPolygon' != 'Polygon'
  

• a gdf containing containing heterogeneous geometries (e.g. Points and Polygons) directly fails in gpd.to_file with the error below:

  ValueError: Geometry column cannot contain mutiple geometry types when writing to file.
  

Suggested improvement

I suggest to support writing GeoDataFrames with multiple geometry types (without having to provide a schema): the inferred schema would just contain a list of geometry types (following Fiona new behaviour);

OGR drivers supporting multiple geometry types would succeed; others would probably fail anyways…

Let me know if there is a case I did not think about.

Ready to contribute

I would be glad to provide a PR if this functionality is desired (I already have some additional unit tests showing the current behaviour)

[amine-aboufirass]

@josemarfdc Indeed it is… I tried it and it works. Thank you very much.

[josemarfdc]

I just wrote this function and tested it for a simple use case. Probably not the best but might be useful to others who have similar problems:

def flatten_gdf_geometry(gdf, geom_type):
    geometry = gdf.geometry
    flattened_geometry = []

    flattened_gdf = geopandas.GeoDataFrame()

    for geom in geometry:
        if geom.type in ['GeometryCollection', 'MultiPoint', 'MultiLineString', 'MultiPolygon']:
            for subgeom in geom:
                if subgeom.type==geom_type:
                    flattened_geometry.append(subgeom)
        else:
            if geom.type==geom_type:
                flattened_geometry.append(geom)

    flattened_gdf.geometry=flattened_geometry

    return flattened_gdf

@awa5114 I believe this functionality is implemented with gdf.explode()