How can I set MESH_OT_vertices_smooth.factor through Python?

How can I set the “Smooth Vertex” smoothing factor MESH_OT_vertices_smooth.factor
through Python?

It can be set in the GUI, after running “Smooth vertex” on an edited object/selected vertices. But I 'd like to do it from Python. (doing the whole smoothing from Python).

bpy.ops.mesh.vertices_smooth(factor=0.5, repeat=1, xaxis=True, yaxis=True, zaxis=True)