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.