Understanding instance and class variable python

Question

Let's suppose I have 2 classes in different scenario.

Scenario 1

class MyClass():
    temp = 5

Scenario 2

class MyClass():
    temp = 5

    def myfunc(self):
          print self.temp

Now when will variable temp will be treated as a class variable and instance variable. I am confused because in both the scenarios I am able to access the value of variable temp using both.

Object.Temp (behaving as instance variable)
ClassName.Temp (behaving as class variable)

I believe similar questions have been asked before but it will be a great help if someone can explain this in context of my question.

mata · Accepted Answer · 2012-06-02 12:22:26Z

5

Class variables are shared between all instances of a class. With immutable types (like int, str, ...) you won't note much of a difference. But consider this:

class MyClass():
    temp = []  
    def myfunc(self, val):
          self.temp.append(val)
          print self.temp

instance1 = MyClass()
instance1.myfunc(1)    # [1]
instance2 = MyClass()
instance2.myfunc(2)    # [1, 2]

In this case both instances share the same list, that is if the instance doesn't have a temp member itself, then that of the class is used.

So if you further do:

MyClass.temp.append(3)
print instance1.temp   # [1, 2, 3]
instance1.temp = []
print instance1.temp   # []         uses the instances temp
print instance2.temp   # [1, 2, 3]
del instance1.temp
print instance1.temp   # [1, 2, 3]  uses the class' temp again

answered Jun 2, 2012 at 12:22

mata

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8 Comments

Lev Levitsky Over a year ago

It's a list, not dict. Great answer, though :)

RanRag Over a year ago

Ok than why there is a need of self in my second scenario to access the temp variable as I understand self just represent the current calling object.

RanRag Over a year ago

@jamylak: right so self is representing object here. So why I am not able to access the value of temp directly inside the function in 2nd scenario why it has to accessed with self.

mata Over a year ago

@Noob temp without self would always refer to a local variable. In python you don't operate automatically on self (like you do in java on this).

RanRag Over a year ago

@mata: thanks for clearing that. I am from java backgroung so I was thinking self and this behaves in the exact same way.

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Community · Accepted Answer · 2017-05-23 12:28:00Z

4

Basically, MyClass.temp is always a class variable. Getting obj.temp returns the class variable, until you try to set obj.temp, which creates a member variable that masks the class variable. I hope this helps:

>>> class MyClass(object):
...     temp = 5
... 
>>> a = MyClass()
>>> b = MyClass()
>>> a.temp
5
>>> b.temp
5
>>> b.temp = 6
>>> a.temp
5
>>> MyClass.temp = 7
>>> a.temp
7
>>> b.temp
6
>>> a.__dict__
{}
>>> b.__dict__
{'temp': 6}
>>> MyClass.__dict__
{..., 'temp': 7}

Edit: As mata says, calling methods (such as append()) on obj.temp does not count as "setting" it.

edited May 23, 2017 at 12:28

CommunityBot

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answered Jun 2, 2012 at 12:21

robert

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Comments

jamylak · Accepted Answer · 2012-06-02 12:25:15Z

1

temp is a class variable. When you access the variable it is searched through the layers of inheritance so since it is not found in the instance itself it checks the class(next layer up) and finds it there.

answered Jun 2, 2012 at 12:25

jamylak

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