Why does @foo.setter in Python not work for me?

You need to use new-style classes which you do by deriving your class from object:

class testDec(object):
   ....

Then it should work.

Just a note for other people who stumble here looking for this exception: both functions need to have the same name. Naming the methods as follows will result in an exception:

@property
def x(self): pass

@x.setter
def x_setter(self, value): pass

Instead give both methods the same name

@property
def x(self): pass

@x.setter
def x(self, value): pass

It is also important to note that the order of the declaration matters. The getter must be defined before the setter in the file or else you will get a NameError: name 'x' is not defined

In case anybody comes here from google, in addition to the above answers I would like to add that this needs careful attention when invoking the setter from the __init__ method of your class based on this answer Specifically:

class testDec(object):                                                                                                                                            

    def __init__(self, value):
        print 'We are in __init__'
        self.x = value # Will call the setter. Note just x here
        #self._x = value # Will not call the setter

    @property
    def x(self):
        print 'called getter'
        return self._x # Note the _x here

    @x.setter
    def x(self, value): 
        print 'called setter'
        self._x = value # Note the _x here

t = testDec(17)
print t.x 

Output:
We are in __init__
called setter
called getter
17

You seem to be using classic old-style classes in python 2. In order for properties to work correctly you need to use new-style classes instead (in python 2 you must inherit from object). Just declare your class as MyClass(object):

class testDec(object):

    @property
    def x(self): 
        print 'called getter'
        return self._x

    @x.setter
    def x(self, value): 
        print 'called setter'
        self._x = value

It works:

>>> k = testDec()
>>> k.x
called getter
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/devel/class_test.py", line 6, in x
    return self._x
AttributeError: 'testDec' object has no attribute '_x'
>>> k.x = 5
called setter
>>> k.x
called getter
5
>>> 

Another detail that might cause problems is that both methods need the same name for the property to work. If you define the setter with a different name like this it won't work:

@x.setter
def x_setter(self, value):
    ...

And one more thing that is not completely easy to spot at first, is the order: The getter must be defined first. If you define the setter first, you get name 'x' is not defined error.