python class instance variables and class variables

This is because of the way Python resolves names with the .. When you write self.list the Python runtime tries to resolve the list name first by looking for it in the instance object, and if it is not found there, then in the class instance.

Let's look into it step by step

self.list.append(1)

1. Is there a list name into the object self?
   • Yes: Use it! Finish.
   • No: Go to 2.
2. Is there a list name into the class instance of object self?
   • Yes: Use it! Finish
   • No: Error!

But when you bind a name things are different:

self.list = []

1. Is there a list name into the object self?
   • Yes: Overwrite it!
   • No: Bind it!

So, that is always an instance variable.

Your first example creates a list into the class instance, as this is the active scope at the time (no self anywhere). But your second example creates a list explicitly in the scope of self.

More interesting would be the example:

class testClass():    list = ['foo']    def __init__(self):        self.list = []        self.list.append('thing')x = testClass()print x.listprint testClass.listdel x.listprint x.list

That will print:

['thing']['foo']['foo']

The moment you delete the instance name the class name is visible through the self reference.

Python has interesting rules about looking up names. If you really want to bend your mind, try this code:

class testClass():    l = []    def __init__(self):        self.l = ['fred']

This will give each instance a variable called l that masks the class variable l. You will still be able to get at the class variable if you do self.__class__.l.

The way I think of it is this... Whenever you do instance.variable (even for method names, they're just variables who's values happen to be functions) it looks it up in the instance's dictionary. And if it can't find it there, it tries to look it up in the instance's class' dictionary. This is only if the variable is being 'read'. If it's being assigned to, it always creates a new entry in the instance dictionary.

In your first example, list is an attribute of the class, shared by all instances of it. This means that you can even access it without having an object of type testClass:

>>> class testClass():...     list = []...     def __init__(self):...             self.list.append("thing")... >>> testClass.list[]>>> testClass.list.append(1)>>> testClass.list[1]

But all objects share the list attribute with the class and each other:

>>> testObject = testClass()>>> testObject.list[1, 'thing']>>> testClass.list[1, 'thing']>>> >>> testObject2 = testClass()>>> testClass.list[1, 'thing', 'thing']>>> testObject2.list[1, 'thing', 'thing']