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

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.xcalled getterTraceback (most recent call last):  File "<stdin>", line 1, in <module>  File "/devel/class_test.py", line 6, in x    return self._xAttributeError: 'testDec' object has no attribute '_x'>>> k.x = 5called setter>>> k.xcalled getter5>>> 

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.setterdef 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.

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:

@propertydef x(self): pass@x.setterdef x_setter(self, value): pass

Instead give both methods the same name

@propertydef x(self): pass@x.setterdef 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

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

class testDec(object):   ....

Then it should work.