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

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Should I use getter and setter in Python?

Getters and Setters in python are often used when: We use getters & setters to add validation logic around getting and setting a value. To avoid direct access of a class field i.e. private variables cannot be accessed directly or modified by external user.

What is the point of @property Python?

Python's property() is the Pythonic way to avoid formal getter and setter methods in your code. This function allows you to turn class attributes into properties or managed attributes. Since property() is a built-in function, you can use it without importing anything.


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.


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


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

class testDec(object):
   ....

Then it should work.


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