How do I add custom property to object in bpy? How do I add a new custom property to a material of an object in my Python script? In an earlier version it was possible to use

bpy.context.object.active_material.gltf_double_sided = True


but I guess in Blender 2.81a it is not supported, the script fails on this line.
Unfortunately, the API documentation is very, confusing. Is there something like, e.g.

bpy.context.object.active_material.data['gltf_double_sided'] = True


(btw, the line above does not work)
The answer is:

bpy.types.Material.gltf_double_sided = bpy.props.BoolProperty()
bpy.context.object.active_material.gltf_double_sided = True


And it works.