Section 2.18. Classes

Classes are declared using the class keyword. A base or parent class is optional; if you do not have one, just use object as the base class. This header line is followed by an optional documentation string, static member declarations, and any method declarations.

class

FooClass(object):
      """my very first class: FooClass"""
      version = 0.1           # class (data) attribute

def

__init__(self, nm='John Doe'):
         """constructor"""
         self.name = nm      # class instance (data) attribute

print

'Created a class instance for', nm

def

showname(self):
         """display instance attribute and class name"""

print

'Your name is', self.name

print

'My name is', self.__class__.__name__

def

showver(self):
         """display class(static) attribute"""

print

self.version  # references FooClass.version

def

addMe2Me(self, x):     # does not use 'self'
         """apply + operation to argument"""

return

x + x

In the above class, we declared one static data type variable version shared among all instances and four methods , __init__() , showname() , showver() , and the familiar addMe2Me() . The show*() methods do not really do much but output the data they were created to output. The __init__() method has a special name, as do all those whose names begin and end with a double underscore ( __ ).

The __init__() method is a function provided by default that is called when a class instance is created, similar to a constructor and called after the object has been instantiated . __init__() can be thought of as a constructor, but unlike constructors in other languages, it does not create an instanceit is really just the first method that is called after your object has been created.

Its purpose is to perform any other type of "start up" necessary for the instance to take on a life of its own. By creating our own __init__() method, we override the default method (which does not do anything) so that we can do customization and other "extra things" when our instance is created. In our case, we initialize a class instance attribute or member called name . This variable is associated only with class instances and is not part of the actual class itself. __ init __ () also features a default argument, introduced in the previous section. You will no doubt also notice the one argument which is part of every method, self .

What is self ? It is basically an instance's handle to itself, the instance on which a method was called. Other OO languages often use an identifier called this .

How to Create Class Instances

>>> foo1 = FooClass()
  Created a class instance for John Doe

The string that is displayed is a result of a call to the __init__() method which we did not explicitly have to make. When an instance is created, __init__() is automatically called, whether we provided our own or the interpreter used the default one.

Creating instances looks just like calling a function and has the exact same syntax. They are both known as "callables." Class instantiation uses the same functional operator as invoking a function or method.

Now that we have successfully created our first class instance, we can make some method calls, too:

>>> foo1.showname()
   Your name is John Doe
   My name is __main__.FooClass
   >>>
   >>> foo1.showver()
   0.1
   >>>

print

foo1.addMe2Me(5)
   10
   >>>

print

foo1.addMe2Me('xyz')
   xyzxyz

>>> foo2 = FooClass('Jane Smith')
   Created a class instance for Jane Smith
   >>> foo2.showname()
   Your name is Jane Smith
   My name is FooClass