confusing about python class and instance variables

Question

I saw the following Python documentation which says that "define variables in a Class" will be class variables:

"Programmer's note: Variables defined in the class definition are class variables; they are shared by all instances. "

but as I wrote sample code like this:

class CustomizedMethods(object):
    class_var1 = 'foo'
    class_var2 = 'bar'

cm1 = CustomizedMethods()
cm2 = CustomizedMethods()
print cm1.class_var1, cm1.class_var2 #'foo bar'
print cm2.class_var1, cm2.class_var2 #'foo bar'
cm2.class_var1, cm2.class_var2 = 'bar','for'
print cm1.class_var1, cm1.class_var2 #'foo bar' #here not changed as my expectation
print cm2.class_var1, cm2.class_var2 #'bar foo' #here has changed but they seemed to become instance variables.

I'm confused since what I tried is different from Python's official documentation.

Answer 1

When you assign an attribute on the instance, it is assigned on the instance, even if it previously existed on the class. At first, class_var1 and class_var2 are indeed class attributes. But when you do cm1.class_var1 = "bar", you are not changing this class attribute. Rather, you are creating a new attribute, also called class_var1, but this one is an instance attribute on the instance cm1.

Here is another example showing the difference, although it still may be a bit tough to grasp:

>>> class A(object):
...     var = []
>>> a = A()
>>> a.var is A.var
True
>>> a.var = []
>>> a.var is A.var
False

At first, a.var is A.var is true (i.e., they are the same object): since a doesn't have it's own attribute called var, trying to access that goes through to the class. After you give a its own instance attribute, it is no longer the same as the one on the class.

Answer 2

You're assigning attributes on the instances, so yes, they become instance variables at that point. Python looks for attributes on whatever object you specify, then if it can't find them there, looks up the inheritance chain (to the class, the class's parents, etc.). So the attribute you assign on the instance "shadows" or "hides" the class's attribute of the same name.