Add Material Example

Add Material.py

class AddMaterial(bpy.types.Operator):
    """Add Material"""
    bl_idname = "object.addmat_operator"
    bl_label = "Add Material"

    def execute(self, context):
        ob = bpy.context.active_object
        bpy.ops.object.shade_smooth()
        
        # Add modifier
        mod_subsurf = ob.modifiers.new("My Modifier", 'SUBSURF')
        mod_subsurf.levels = 3
        bpy.ops.object.modifier_apply(modifier="My Modifier")
      
        # Add material
        new_mat = bpy.data.materials.new("New Material")
        new_mat.use_nodes = True
        principled = new_mat.node_tree.nodes['Principled BSDF']
        principled.inputs['Metallic'].default_value = 1
        principled.inputs['Roughness'].default_value = 0
        principled.inputs['Base Color'].default_value = (1,0,0,0)
        ob.data.materials.append(new_mat)
        return {'FINISHED'}
    
def register():
    bpy.utils.register_class(AddMaterial)


def unregister():
    bpy.utils.unregister_class(AddMaterial)
    
    
class MyPanel(bpy.types.Panel):
    
    bl_label = "Panel"
    bl_idname = "MY_PT_Panel"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_category = "Add Material"

    def draw(self, context):
        layout = self.layout

        obj = context.object

        row = layout.row()
        row.operator("object.addmat_operator", text = "Add Material")

classes = (AddMaterial, MyPanel)

def register():
    for cls in classes:
        bpy.utils.register_class(cls)

def unregister():
    for cls in classes:
        bpy.utils.unregister_class(cls)

if __name__ == "__main__":
    register()