Static instance variables in python

Question

I am trying to learn about static variables in python. According to this answer this piece of code should make i unique for all the objects of class Test

The code:

>>> class Test(object):
...     _i = 3
...     @property
...     def i(self):
...             return self._i
...     @i.setter
...     def i(self,val):
...             self._i = val
... 
>>> 
>>> x1 =  Test()
>>> x1.i
3
>>> x2 = Test()
>>> x2.i
3
>>> x1.i = 10
>>> x1.i
10
>>> x2.i
3

But as you can see object x1.i is not equal to x2.i.
I tried to do this in both python 2.7 and 3.4 but the result is same. I think my understanding about this concept is wrong.

Could somebody please explain this to me or guide me to a resource.

They are unique as you expected in your second sentence. What exactly is your question? — Selcuk
– Selcuk, Commented Jun 1, 2016 at 8:33
That linked answer has it wrong. self._i = val will assign to a local to that instance attribute _i (which then shadows the class attr), not the class attribute. — Ilja Everilä
– Ilja Everilä, Commented Jun 1, 2016 at 8:35
@Selcuk in the linked answer it mentions that x1.i is equal to x2.i, which does not happen when I tried to do the same. my question is if I am wrong or the mentioned answer is misleading. — Nagri
– Nagri, Commented Jun 1, 2016 at 8:41

Community · Accepted Answer · 2017-05-23 12:15:54Z

1

As noted, the linked answer gets it wrong.

class Test(object):
    i = 0


t = Test()
t.i = 4
print t.i
>>> 4
print t.__class__.i
>>> 0

Fields declared on the class are looked up when not found on the instance, but you cannot modify the class directly via self. self.i = 4 always modifies the instance. You can do stuff like

class Test(object):
    _i = 0

    @property
    def i(self):
        return self._i

    @i.setter
    def i(self, value):
        Test._i = value

But duplicating the class name all over the place does not seem a good idea and self.__class__ does not play too well with inheritance. This also completely hides the fact that i was initially a class variable which may or may not be a good idea, either.

edited May 23, 2017 at 12:15

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answered Jun 1, 2016 at 8:43

dhke

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2 Comments

Daniel Roseman Over a year ago

I'm not sure what you mean by "Fields declared on the class are copied to self's dictionary", but it doesn't sound right. i is not in the instance dict until you set it explicitly on t; rather, when Python resolves the name it falls back to the class-level attribute if it is not found at instance level.

dhke Over a year ago

@DanielRoseman Hmm. It seems I fell for dir()'s magic, here (it shows i). Corrected.

Dmitry Erohin · Accepted Answer · 2016-06-01 08:48:28Z

0

class Test(object):

    _i = 3

    @property
    def i(self):
        return self._i

    @i.setter
    def i(self,val):
        self._i = val

if __name__ == '__main__':

    x1 =  Test()
    x2 = Test()

    x1.i = 10
    print(x1.__dict__)
    print(Test.__dict__)

{'_i': 10}
{'i': <property object at 0x02551180>, '__weakref__': <attribute '__weakref__' of 'Test' objects>, '_i': 3, '__module__': '__main__', '__doc__': None, '__dict__': <attribute '__dict__' of 'Test' objects>}

x1.i = 10 create values object

answered Jun 1, 2016 at 8:48

Dmitry Erohin

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