blender/doc/python_api/examples/bpy.props.4.py

"""
Update Example
++++++++++++++

It can be useful to perform an action when a property is changed and can be
used to update other properties or synchronize with external data.

All properties define update functions except for CollectionProperty.
"""

import bpy


def update_func(self, context):
    print("my test function", self)

bpy.types.Scene.testprop = bpy.props.FloatProperty(update=update_func)

bpy.context.scene.testprop = 11.0

# >>> my test function <bpy_struct, Scene("Scene")>