Why does @foo.setter in Python not work for me?
PythonDecoratorNew Style-ClassPython Problem Overview
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?
Python Solutions
Solution 1 - Python
You seem to be using [classic old-style classes][1] in python 2. In order for [properties][2] to work correctly you need to use [new-style classes][3] instead (in python 2 you must [inherit from object
][4]). 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.
[1]: https://docs.python.org/2/glossary.html#term-classic-class "term-classic-class" [2]: https://docs.python.org/2/library/functions.html#property "property" [3]: https://docs.python.org/2/glossary.html#term-new-style-class "term-new-style-class" [4]: https://docs.python.org/2/reference/datamodel.html#newstyle "newstyle"
Solution 2 - Python
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
Solution 3 - Python
You need to use new-style classes which you do by deriving your class from object:
class testDec(object):
....
Then it should work.
Solution 4 - Python
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