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

StackOverflow

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 rating: 318

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.





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