python class instance variables and class variables

Question

I'm having a problem understanding how class / instance variables work in Python. I don't understand why when I try this code the list variable seems to be a class variable

class testClass():
    list = []
    def __init__(self):
        self.list.append('thing')

p = testClass()
print p.list

f = testClass()
print f.list

Output:

['thing']
['thing', 'thing']

and when I do this it seems to be an instance variable

class testClass():
    def __init__(self):
        self.list = []
        self.list.append('thing')

p = testClass()
print p.list

f = testClass()
print f.list

Output:

['thing']
['thing']

Answer 1

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.list
print testClass.list
del x.list
print x.list

That will print:

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

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