How to set coordinate dimensions in Django GDAL API?

The Django GDAL documentation for coord_dim says:

coord_dim
Returns or sets the coordinate dimension of this geometry. For example, the value would be 2 for two-dimensional geometries.

But how do I actually set the dimension? I’m working with three dimensional data, and trying to convert it to two dimensional data:

    ds = DataSource(options['filename'])
    for layer in ds:
        for i in layer:
            print i.geom.coord_dim
            i.geom.coord_dim = 2
            print i.geom.coord_dim

This doesn’t produce an error, but outputs 3 twice, and doesn’t seem to change the coordinates at all when I print those.

How do I use coord_dim to set the number of coordinates?

The underlying thing I’m trying to achieve is to transform three-dimensional data to two-dimensional data.