How should I refer to class variables in Python?

Question

In Python I can refer to a class level variable with either ClassName.class_var or self.class_var from within a class. Now as long as I do not override the class level variables name within an object by creating an instance variable of the same name (which I find would be kind out silly), they will always refer to the same thing.

My question is, which is a better practice? or more common?

I see coding books and source code itself using either way. In some cases, I see them using BOTH methods to refer to the variable within the same class which I think is really inconsistent.

Here I provide an example :

class Foo(object) :
    class_var = ['same','for','all']
    def __init__(self, instance_var) :
        self.instance_var = instance_var
    def print_words(self) :
        # here I could use self.class_var
        for w in Foo.class_var :
            print w

Jason S · Accepted Answer · 2016-07-02 03:59:26Z

12

If you have only the one class and you only read the variable, you won't see a lot of difference.

If you take into account the possibility of subclasses that override class_var, then self.class_var will look in the current class first while Foo.class_var will continue to refer concretely to that particular value. The best practice depends on your intent but since using classes in a language that makes them optional implies at least some interest in polymorphism, self is probably more generally useful.

Also, if you want to set the value, self.whatever = will set an instance variable and Foo.whatever = will set the class variable which could then affect any other instance as well.

answered Jul 2, 2016 at 3:59

Jason S

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Jeff · Accepted Answer · 2016-07-02 04:16:08Z

1

The problem with using Foo.class_var from your example is in inheritance. You can see it illustrated here, beginning with your Foo() class:

class Bar(Foo):
    class_var = ['other','one','here']

In [2]: a = Foo(['one'])

In [3]: b = Bar(['one'])

In [4]: a.class_var
Out[4]: ['same', 'for', 'all']

In [5]: b.class_var
Out[5]: ['other', 'one', 'here']

In [6]: b.print_words()
same
for
all

It's possible this is the behavior you want, but I think it's uncommon. You're usually going to run into the self variety.

answered Jul 2, 2016 at 4:16

Jeff

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