Monkey patching a @property

from module import ClassToPatch

def get_foo(self):
    return 'foo'

setattr(ClassToPatch, 'foo', property(get_foo))

Subclass the base class (Foo) and change single instance's class to match the new subclass using __class__ attribute:

>>> class Foo:
...     @property
...     def bar(self):
...         return 'Foo.bar'
...
>>> f = Foo()
>>> f.bar
'Foo.bar'
>>> class _SubFoo(Foo):
...     bar = 0
...
>>> f.__class__ = _SubFoo
>>> f.bar
0
>>> f.bar = 42
>>> f.bar
42