Chapter 6. OOP

-Classes and Objects

In the previous sections we learnt about functions which provide certain level -of abstraction to our code by holding the code which performs one or more -specific functionalities. We were able to use this function as many times as we -wanted. In addition to functions, Python also higher level of abstractions -through Classes and Objects. Objects can be loosely defined as a -collection of a set of data items and a set of methods. The data items can be -any valid Python variable or any Python object. Functions enclosed within a class -are called as methods. If you are thinking if methods are functions why is there -a distinction between the two? The answer to this will be given as we walk through -the concepts of Classes and Objects. Classes contain the definition for the -Objects. Objects are instances of Classes.

A class is defined using the keyword class followed by the class name, in -turn followed by a semicolon. The statements that a Class encloses are written -in a new block, i.e on the next indentation level:

 class Employee:
-  def setName(self, name):
-    self.name = name
-
-  def getName(self):
-    return self.name

In the above example, we defined a class with the name Employee. We also defined -two methods, setName and getName for this class. It is important to note the -differences between the normal Python functions and class methods defined above. -Each method of the class must take the same instance of the class(object) from -which it was called as the first argument. It is conventionally given the name, -self. Note that self is only a convention. You can use any other name, but -the first argument to the method will always be the same object of the class -from which the method was called. The data memebers that belong to the class are -called as class attributes. Class attributes are preceded by the object of -the class and a dot. In the above example, name is a class attribute since it -is preceded by the self object. Class attributes can be accessed from -anywhere within the class.

We can create objects of a class outside the class definition by using the same -syntax we use to call a function with no parameters. We can assign this object -to a variable:

 emp = Employee()

In the above example, we create an object named emp of the class Employee. -All the attributes and methods of the class can be accessed by the object of the -class using the standard notation object.attribute or object.method(). -Although the first parameter of a class method is the self object, it must not -be passed as an argument when calling the method. The self object is implicitly -passed to the method by the Python interpreter. All other arguments passing rules -like default arguments, keyword arguments, argument packing and unpacking follow -the same rules as those for ordinary Python functions:

 >>> emp.setName('John')
->>> name = emp.getName()
->>> print name
-John
->>> print emp.name
-John

If we at all try to access a class attribute before assigning a value to it, i.e -before creating it, Python raises the same error as it would raise for the -accessing undefined variable:

 >>> emp = Employee()
->>> emp.name
-Traceback (most recent call last):
-  File "class.py", line 10, in <module>
-    print e.name
-AttributeError: Employee instance has no attribute 'name'