Core Python Programming (2nd Edition)

13.4. Class Attributes
What is an attribute? An attribute is a data or functional element that belongs to another object and is
accessed via the familiar dotted-attribute notation. Some Python types such as complex numbers have
data attributes (real and imag), while others such as lists and dictionaries have methods (functional
attributes).
One interesting side note about attributes is that when you are accessing an attribute, it is also an
object and may have attributes of its own which you can then access, leading to a chain of attributes, i.
e., myThing.subThing.subSubThing, etc. Some familiar examples are:
● sys.stdout.write('foo')
● print myModule.myClass.__doc__
● myList.extend(map(upper, open('x').readlines()))
Class attributes are tied only to the classes in which they are defined, and since instance objects are the
most commonly used objects in everyday OOP, instance data attributes are the primary data attributes
you will be using. Class data attributes are useful only when a more "static" data type is required which
is independent of any instances, hence the reason we are making the next section advanced, optional
reading. (If you are unfamiliar with static, it just means a value that hangs around a function for each
call, or a piece of data in a class that is the same across all instances. More about static data in the next
subsection.)
In the succeeding subsection, we will briefly describe how methods in Python are implemented and
invoked. In general, all methods in Python have the same restriction: they require an instance before
they can be called.
13.4.1. Class Data Attributes
Data attributes are simply variables of the class we are defining. They can be used like any other
variable in that they are set when the class is created and can be updated either by methods within the
class or elsewhere in the main part of the program.
Such attributes are better known to OO programmers as static members, class variables, or static data.
They represent data that is tied to the class object they belong to and are independent of any class
instances. If you are a Java or C++ programmer, this type of data is the same as placing the static
keyword in front of a variable declaration.
Static members are generally used only to track values associated with classes. In most circumstances,
you would be using instance attributes rather than class attributes. We will compare the differences
between class and instance attributes when we formally introduce instances.
Here is an example of using a class data attribute (foo):
>>> class C(object):
... foo = 100
>>> print C.foo