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what is wrong with this property setter

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  what is wrong with this property setter Nagy László Zsolt <gandalf@shopzeus.com> - 2016-06-09 09:28 +0200
    Re: what is wrong with this property setter Mark Summerfield <list@qtrac.plus.com> - 2016-06-09 00:49 -0700
    Re: what is wrong with this property setter Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-06-09 17:52 +1000

#109702 — what is wrong with this property setter

class Test:
    def __init__(self):
        self._parent = None

    @property
    def parent(self):
        return self._parent

    @parent.setter
    def set_parent(self, new_parent):
        self._parent = new_parent


p, c = Test(), Test()
c.parent = p

>py -3 test.py

Traceback (most recent call last):
  File "test.py", line 15, in <module>
    c.parent = p
AttributeError: can't set attribute

BTW this does work, but it is not that elegant:

class Test:
    def __init__(self):
        self._parent = None

    def get_parent(self):
        return self._parent

    def set_parent(self, new_parent):
        self._parent = new_parent

    parent = property(get_parent, set_parent)


p, c = Test(), Test()
c.parent = p

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#109705

On Thursday, June 9, 2016 at 8:28:47 AM UTC+1, Nagy László Zsolt wrote:
> class Test:
>     def __init__(self):
>         self._parent = None
> 
>     @property
>     def parent(self):
>         return self._parent
> 
>     @parent.setter
>     def set_parent(self, new_parent):
>         self._parent = new_parent
> 
> 
> p, c = Test(), Test()
> c.parent = p
> 
> >py -3 test.py
> 
> Traceback (most recent call last):
>   File "test.py", line 15, in <module>
>     c.parent = p
> AttributeError: can't set attribute
> 
> BTW this does work, but it is not that elegant:
> 
> class Test:
>     def __init__(self):
>         self._parent = None
> 
>     def get_parent(self):
>         return self._parent
> 
>     def set_parent(self, new_parent):
>         self._parent = new_parent
> 
>     parent = property(get_parent, set_parent)
> 
> 
> p, c = Test(), Test()
> c.parent = p

Change the name of the setter from set_parent to parent, i.e.,

@parent.setter
def parent(self, new_parent):
   ...

That works for me on Python 3.4.

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#109707

On Thursday 09 June 2016 17:28, Nagy László Zsolt wrote:

> class Test:

Are you using Python 3 or 2? In Python 2, property doesn't work correctly with 
classes unless they inherit from object (directly or indirectly).


>     def __init__(self):
>         self._parent = None
> 
>     @property
>     def parent(self):
>         return self._parent
> 
>     @parent.setter
>     def set_parent(self, new_parent):
>         self._parent = new_parent

If you use the "setter" method, you must use the same name, in this case 
"parent", for each part of the property.

If you want to give your getters and setters different names, you have to use 
the older property API:

class Test(object):
    def get_parent(self): ...
    def set_parent(self, value): ...
    parent = property(get_parent, set_parent)


The reason why your code doesn't work is because of the way decorator syntax is 
defined. You have:

@property
def parent(self): ...

which becomes:

parent = property(parent)

Then you have:

@parent.setter
def set_parent(self, new_parent):

which becomes:

set_parent = parent.setter(set_parent)


which leaves you with TWO property objects, not one:

(1) parent has a getter but no setter;

(2) set_parent has a getter and setter (I think).



-- 
Steve

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