object_oriented

= Python - Object Oriented =

  



Python has been an object-oriented language from day one. Because of this, creating and using classes and objects are downright easy. This chapter helps you become an expert in using Python's object-oriented programming support. If you don't have any previous experience with object-oriented (OO) programming, you may want to consult an introductory course on it or at least a tutorial of some sort so that you have a grasp of the basic concepts. However, here is small introduction of Object-Oriented Programming (OOP) to bring you at speed: = Overview of OOP Terminology = **Class:** A user-defined prototype for an object that defines a set of attributes that characterize any object of the class. The attributes are data members (class variables and instance variables) and methods, accessed via dot notation. **Class variable:** A variable that is shared by all instances of a class. Class variables are defined within a class but outside any of the class's methods. Class variables aren't used as frequently as instance variables are. **Data member:** A class variable or instance variable that holds data associated with a class and its objects. **Function overloading:** The assignment of more than one behavior to a particular function. The operation performed varies by the types of objects (arguments) involved. **Instance variable:** A variable that is defined inside a method and belongs only to the current instance of a class. **Inheritance :** The transfer of the characteristics of a class to other classes that are derived from it. **Instance:** An individual object of a certain class. An object obj that belongs to a class Circle, for example, is an instance of the class Circle. **Instantiation :** The creation of an instance of a class. **Method :** A special kind of function that is defined in a class definition. <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">**Object :** A unique instance of a data structure that's defined by its class. An object comprises both data members (class variables and instance variables) and methods. <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">**Operator overloading:** The assignment of more than one function to a particular operator. = Creating Classes: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">The //class// statement creates a new class definition. The name of the class immediately follows the keyword //class// followed by a colon as follows: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">class ClassName: 'Optional class documentation string' class_suite code || > >
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> The class has a documentation string which can be access via //ClassName.__doc__//.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> The //class_suite// consists of all the component statements, defining class members, data attributes, and functions.

Example:
<span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Following is the example of a simple Python class: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">class Employee: 'Common base class for all employees' empCount = 0

def __init__(self, name, salary): self.name = name self.salary = salary Employee.empCount += 1

def displayCount(self): print "Total Employee %d" % Employee.empCount

def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary code || > > > = Creating instance objects: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">To create instances of a class, you call the class using class name and pass in whatever arguments its //__init__// method accepts. code <span style="font-family: 'Courier New',monospace; font-size: 12px;">"This would create first object of Employee class" emp1 = Employee("Zara", 2000) "This would create second object of Employee class" emp2 = Employee("Manni", 5000) code || = Accessing attributes: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">You access the object's attributes using the dot operator with object. Class variable would be accessed using class name as follows: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">emp1.displayEmployee emp2.displayEmployee print "Total Employee %d" % Employee.empCount code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Now putting it all together: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> The variable //empCount// is a class variable whose value would be shared among all instances of a this class. This can be accessed as //Employee.empCount// from inside the class or outside the class.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> The first method //__init__// is a special method which is called class constructor or initialization method that Python calls when you create a new instance of this class.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> You declare other class methods like normal functions with the exception that the first argument to each method is //self//. Python adds the //self// argument to the list for you; you don't need to include it when you call the methods.

class Employee: 'Common base class for all employees' empCount = 0

def __init__(self, name, salary): self.name = name self.salary = salary Employee.empCount += 1

def displayCount(self): print "Total Employee %d" % Employee.empCount

def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary

"This would create first object of Employee class" emp1 = Employee("Zara", 2000) "This would create second object of Employee class" emp2 = Employee("Manni", 5000) emp1.displayEmployee emp2.displayEmployee print "Total Employee %d" % Employee.empCount code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This would produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">Name : Zara ,Salary:  2000 Name : Manni ,Salary:  5000 Total Employee 2 code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">You can add, remove, or modify attributes of classes and objects at any time: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">emp1.age = 7 # Add an 'age' attribute. emp1.age = 8 # Modify 'age' attribute. del emp1.age # Delete 'age' attribute. code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Instead of using the normal statements to access attributes, you can use following functions: > > > > code <span style="font-family: 'Courier New',monospace; font-size: 12px;">hasattr(emp1, 'age')   # Returns true if 'age' attribute exists getattr(emp1, 'age')   # Returns value of 'age' attribute setattr(emp1, 'age', 8) # Set attribute 'age' at 8 delattr(empl, 'age')   # Delete attribute 'age' code || = Built-In Class Attributes: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Every Python class keeps following built-in attributes and they can be accessed using dot operator like any other attribute: > > > > > <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">For the above class let's try to access all these attributes: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">print "Employee.__doc__:", Employee.__doc__ print "Employee.__name__:", Employee.__name__ print "Employee.__module__:", Employee.__module__ print "Employee.__bases__:", Employee.__bases__ print "Employee.__dict__:", Employee.__dict__ code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This would produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">Employee.__doc__: Common base class for all employees Employee.__name__: Employee Employee.__module__: __main__ Employee.__bases__: Employee.__dict__: {'__module__': '__main__', 'displayCount': <function displayCount at 0xb7c84994>, 'empCount': 2, 'displayEmployee': <function displayEmployee at 0xb7c8441c>, '__doc__': 'Common base class for all employees', '__init__': <function __init__ at 0xb7c846bc>} code || = Destroying Objects (Garbage Collection): = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Python deletes unneeded objects (built-in types or class instances) automatically to free memory space. The process by which Python periodically reclaims blocks of memory that no longer are in use is termed garbage collection. <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Python's garbage collector runs during program execution and is triggered when an object's reference count reaches zero. An object's reference count changes as the number of aliases that point to it changes: <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">An object's reference count increases when it's assigned a new name or placed in a container (list, tuple, or dictionary). The object's reference count decreases when it's deleted with //del//, its reference is reassigned, or its reference goes out of scope. When an object's reference count reaches zero, Python collects it automatically. code <span style="font-family: 'Courier New',monospace; font-size: 12px;">a = 40     # Create object <40> b = a      # Increase ref. count of <40> c = [b]    # Increase ref. count of <40>
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> The **getattr(obj, name[, default])** : to access the attribute of object.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> The **hasattr(obj,name)** : to check if an attribute exists or not.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> The **setattr(obj,name,value)** : to set an attribute. If attribute does not exist then it would be created.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> The **delattr(obj, name)** : to delete an attribute.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> **__dict__ :** Dictionary containing the class's namespace.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> **__doc__ :** Class documentation string, or None if undefined.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> **__name__:** Class name.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> **__module__:** Module name in which the class is defined. This attribute is "__main__" in interactive mode.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> **__bases__ :** A possibly empty tuple containing the base classes, in the order of their occurrence in the base class list.

del a      # Decrease ref. count of <40> b = 100    # Decrease ref. count of <40> c[0] = -1  # Decrease ref. count of <40> code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">You normally won't notice when the garbage collector destroys an orphaned instance and reclaims its space. But a class can implement the special method //__del__//, called a destructor, that is invoked when the instance is about to be destroyed. This method might be used to clean up any nonmemory resources used by an instance.

Example:
<span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This __del__ destructor prints the class name of an instance that is about to be destroyed: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python

class Point: def __init( self, x=0, y=0): self.x = x     self.y = y   def __del__(self): class_name = self.__class__.__name__ print class_name, "destroyed"

pt1 = Point pt2 = pt1 pt3 = pt1 print id(pt1), id(pt2), id(pt3) # prints the ids of the obejcts del pt1 del pt2 del pt3 code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This would produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">3083401324 3083401324 3083401324 Point destroyed code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">**Note:** Ideally, you should define your classes in separate file then you should import them in your main program file using //import// statement. Kindly check [|Python - Modules] chapter for more detail on importing modules and classes. = Class Inheritance: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Instead of starting from scratch, you can create a class by deriving it from a preexisting class by listing the parent class in parentheses after the new class name: <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">The child class inherits the attributes of its parent class, and you can use those attributes as if they were defined in the child class. A child class can also override data members and methods from the parent.

Syntax:
<span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Derived classes are declared much like their parent class; however, a list of base classes to inherit from are given after the class name: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">class SubClassName (ParentClass1[, ParentClass2, ...]): 'Optional class documentation string' class_suite code ||

Example:
code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python

class Parent:       # define parent class parentAttr = 100 def __init__(self): print "Calling parent constructor"

def parentMethod(self): print 'Calling parent method'

def setAttr(self, attr): Parent.parentAttr = attr

def getAttr(self): print "Parent attribute :", Parent.parentAttr

class Child(Parent): # define child class def __init__(self): print "Calling child constructor"

def childMethod(self): print 'Calling child method'

c = Child         # instance of child c.childMethod     # child calls its method c.parentMethod    # calls parent's method c.setAttr(200)      # again call parent's method c.getAttr         # again call parent's method code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This would produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">Calling child constructor Calling child method Calling parent method Parent attribute : 200 code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Similar way you can drive a class from multiple parent classes as follows: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">class A:       # define your class A .....

class B:        # define your calss B .....

class C(A, B):  # subclass of A and B ..... code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">You can use issubclass or isinstance functions to check a relationships of two classes and instances: > > = Overriding Methods: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">You can always override your parent class methods. One reason for overriding parent's methods is because you may want special or different functionality in your subclass.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> The **issubclass(sub, sup)** boolean function returns true if the given subclass **sub** is indeed a subclass of the superclass **sup**.
 * <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> The **isinstance(obj, Class)** boolean function returns true if //obj// is an instance of class//Class// or is an instance of a subclass of Class

Example:
code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python

class Parent:       # define parent class def myMethod(self): print 'Calling parent method'

class Child(Parent): # define child class def myMethod(self): print 'Calling child method'

c = Child         # instance of child c.myMethod        # child calls overridden method code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This would produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">Calling child method code || = Base Overloading Methods: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Following table lists some generic functionality that you can override in your own classes: Constructor (with any optional arguments) Sample Call : //obj = className(args)// || Destructor, deletes an object Sample Call : //dell obj// || Evaluatable string representation Sample Call : //repr(obj)// || Printable string representation Sample Call : //str(obj)// || Object comparison Sample Call : //cmp(obj, x)// || = Overloading Operators: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Suppose you've created a Vector class to represent two-dimensional vectors. What happens when you use the plus operator to add them? Most likely Python will yell at you. <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">You could, however, define the //__add__// method in your class to perform vector addition, and then the plus operator would behave as per expectation:
 * ~ SN ||~ Method, Description & Sample Call ||
 * 1 || **__init__ ( self [,args...] )**
 * 2 || **__del__( self )**
 * 3 || **__repr__( self )**
 * 4 || **__str__( self )**
 * 5 || **__cmp__ ( self, x )**

Example:
code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python

class Vector: def __init__(self, a, b): self.a = a     self.b = b

def __str__(self): return 'Vector (%d, %d)' % (self.a, self.b)

def __add__(self,other): return Vector(self.a + other.a, self.b + other.b)

v1 = Vector(2,10) v2 = Vector(5,-2) print v1 + v2 code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This would produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">Vector(7,8) code || = Data Hiding: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">An object's attributes may or may not be visible outside the class definition. For these cases, you can name attributes with a double underscore prefix, and those attributes will not be directly visible to outsiders:

Example:
code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python

class JustCounter: __secretCount = 0

def count(self): self.__secretCount += 1 print self.__secretCount

counter = JustCounter counter.count counter.count print counter.__secretCount code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This would produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">1 2 Traceback (most recent call last): File "test.py", line 12, in    print counter.__secretCount code AttributeError: JustCounter instance has no attribute '__secretCount' || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Python protects those members by internally changing the name to include the class name. You can access such attributes as //object._className__attrName//. <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">If you would replace your last line as following, then it would work for you: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">......................... print counter._JustCounter__secretCount code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This would produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">1 2 2 code ||

<span style="background-color: #ffffff; color: #900b09; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> <span style="background-color: #ffffff; color: #900b09; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> <span style="background-color: #ffffff; color: #900b09; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;">