Beginning Python - From Novice to Professional

152 CHAPTER 7 ■ MORE ABSTRACTION
Both create a function that returns the square of its argument, and both bind the variable
foo to that function. The name foo may be defined in the global (module) scope, or it may be
local to some function or method. The same thing happens when you define a class; all the
code in the class statement is executed in a special namespace—the class namespace. This
namespace is accessible later by all members of the class. Not all Python programmers know
that class definitions are simply code sections that are executed, but it can be useful information.
For example, you aren’t restricted to def statements:
>>> class C:
print 'Class C being defined...'
Class C being defined...
>>>
Okay, that was a bit silly. But consider the following:
class MemberCounter:
members = 0
def init(self):
MemberCounter.members += 1
>>> m1 = MemberCounter()
>>> m1.init()
>>> MemberCounter.members
1
>>> m2 = MemberCounter()
>>> m2.init()
>>> MemberCounter.members
2
In the preceding code, a variable is defined in the class scope, which can be accessed by all
the members (instances), in this case to count the number of class members. Note the use of
init to initialize all the instances: I’ll automate that in Chapter 9.
This class scope variable is accessible from every instance as well, just as methods are:
>>> m1.members
2
>>> m2.members
2
What happens when you rebind the members attribute in an instance?
>>> m1.members = 'Two'
>>> m1.members
'Two'
>>> m2.members
2
The new members value has been written into an attribute in m1, shadowing the classwide
variable. This mirrors the behavior of local and global variables.