[Juhana Jauhiainen]

Python class and instance variables

Confusing class variables and instance variables in Python is a pretty common mistake. Or.. atleast I've made it more than once. Python doesn't use static keyword but instead the location the variables are defined in to differentiate between the two which will seem pretty foreign to people coming from other object oriented languages.

Here's an example of how you would declare class variables in Python

class Foo:
bar = True
instance = Foo()

bar is now accessible by Foo.bar, self.bar inside the methods of class Foo and with instance.bar using a instance of the class.

Now here's an example how to declare a instance variable in Python.

class Foo:
def __init__(self):
self.bar = True
instance = Foo()

bar is now accessible with self.bar inside class Foo methods and instance.bar using a instance of the class.

So what's the difference between these two? In addition to the already mentioned access rules, class variables are shared between all instances of the class while instance variables are unique to the instance.

If this was not the intention, it can lead to hard to trace bugs.

Coming from other object oriented programming languages you might
think you need to declare the variable first and then initialize it in __init__.

class Foo:
bar = False
def __init__(self, set_bar):
self.bar = set_bar

This is however not necessary and while it probably won't cause issues, because the instance variable hides the class variable, it's not very Pythonic.