Question

So, I’m playing with decorators in Python 2.6, and I’m having some trouble getting them to work. Here is my class file:

class testDec:      @property     def x(self):          print 'called getter'         return self._x      @x.setter     def x(self, value):          print 'called setter'         self._x = value 

What I thought this meant is to treat x like a property, but call these functions on get and set. So, I fired up IDLE and checked it:

>>> from testDec import testDec from testDec import testDec >>> t = testDec() t = testDec() >>> t.x t.x called getter Traceback (most recent call last):   File '<stdin>', line 1, in <module>   File 'testDec.py', line 18, in x     return self._x AttributeError: testDec instance has no attribute '_x' >>> t.x = 5 t.x = 5 >>> t.x t.x 5 

Clearly the first call works as expected, since I call the getter, and there is no default value, and it fails. OK, good, I understand. However, the call to assign t.x = 5 seems to create a new property x, and now the getter doesn’t work!

What am I missing?

Answer 1

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.