Blender property and callbacks

b3d_prop_callbacks.py

"""
Example of callbacks used for Blender integer property
"""

import bpy

# Some global values
values = {}
default_value = -1

# Callback function called, when property is changed
def cb_update(self, context):
    print('cb_update() my_prop, value: ', self.name, self.my_prop, values)
    return None

# Callback function called, when value is requested using obj.my_prop.
# Note: value of property is generated from other source (dictonary in this
# case).
def cb_get(self):
    # Do not call self.my_prop inside this callback function, because it will
    # cause infinity recursive calling of this function!
    print('cb_get() my_prop, value: ', self.name)
    try:
        value = values[self.name]
    except KeyError:
        value = default_value
    return value

# Callback function called, when new value of property is set using:
# obj.my_prop = 17. Note:  Use this function, when you want to
# control the value of property and save it somewhere else (dictionary
# in this case).
def cb_set(self, value):
    print('cb_set() my_prop, value: ', self.name, self.my_prop, value)
    # Do not do this: self.my_prop = value, because it will cause
    # recursive calling of this callback function
    values[self.name] = value
    return None

# Create new property in object
bpy.types.Object.my_prop = bpy.props.IntProperty(name="MyProp",
    default=-1,
    description="My Property",
    update=cb_update,
    get=cb_get,
    set=cb_set
    )

# Simple tests of callback functions
print(bpy.context.active_object.my_prop)
bpy.context.active_object.my_prop = 10
print(bpy.context.active_object.my_prop)

This solution is not perfect. There will be serious problem, when e.g.: blend file will be reloaded. There are some useful notes at Blender stack exchange: http://blender.stackexchange.com/questions/27361/duplicating-object-and-default-value-of-property