settertest.py

import datetime

class Base(object):
        pass

class Foo(Base):
        name = None
        _modified = None

        @property
        def modified(self):
                return self._modified;

        @modified.setter
        def modified(self, value):
                self._modified = value

        def __setattr__(self, name, value):
                Base.__setattr__(self, name, value)
                if name != '_modified':
                        Base.__setattr__(self, '_modified', datetime.datetime.now())


foo = Foo()

print(foo._modified)

foo.name = "Bar"

print(foo._modified)