Python has been an object-oriented language since it existed. 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 do not 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 are not 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 or 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.
- 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.
- Operator overloading − The assignment of more than one function to a particular operator.
Creating Classes
The class statement creates a new class definition. The name of the class immediately follows the keyword class followed by a colon as follows −
class ClassName: 'Optional class documentation string' class_suite
The class has a documentation string, which can be accessed via ClassName.__doc__.
The class_suite consists of all the component statements defining class members, data attributes and functions.
Example
Following is the example of a simple Python class −
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
Creating Instance Objects
To create instances of a class, you call the class using class name and pass in whatever arguments its __init__ method accepts.
"This would create first object of Employee class" emp1 = Employee("Zara", 2000) "This would create second object of Employee class" emp2 = Employee("Manni", 5000)
Accessing Attributes
You access the object's attributes using the dot operator with object. Class variable would be accessed using class name as follows −
emp1.displayEmployee() emp2.displayEmployee() print "Total Employee %d" % Employee.empCount
Now, putting all the concepts together −
#!/usr/bin/python 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
When the above code is executed, it produces the following result −
Name : Zara ,Salary: 2000 Name : Manni ,Salary: 5000 Total Employee 2
Built-In Class Attributes
Every Python class keeps following built-in attributes and they can be accessed using dot operator like any other attribute −
__dict__ − Dictionary containing the class's namespace.
__doc__ − Class documentation string or none, if undefined.
__name__ − Class name.
__module__ − Module name in which the class is defined. This attribute is "__main__" in interactive mode.
__bases__ − A possibly empty tuple containing the base classes, in the order of their occurrence in the base class list.
For the above class let us try to access all these attributes −
#!/usr/bin/python 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 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__
When the above code is executed, it produces the following result −
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>}
Class Inheritance
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.
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
Derived classes are declared much like their parent class; however, a list of base classes to inherit from is given after the class name −
class SubClassName (ParentClass1[, ParentClass2, ...]): 'Optional class documentation string' class_suite
Overriding Methods
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.
Example
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
When the above code is executed, it produces the following result −
Calling child method
Base Overloading Methods
Following table lists some generic functionality that you can override in your own classes −
Sr.No. | Method, Description & Sample Call |
---|---|
1 | __init__ ( self [,args...] ) Constructor (with any optional arguments) Sample Call : obj = className(args) |
2 | __del__( self ) Destructor, deletes an object Sample Call : del obj |
3 | __repr__( self ) Evaluable string representation Sample Call : repr(obj) |
4 | __str__( self ) Printable string representation Sample Call : str(obj) |
5 | __cmp__ ( self, x ) Object comparison Sample Call : cmp(obj, x) |
Overloading Operators
Suppose you have 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.
You could, however, define the __add__ method in your class to perform vector addition and then the plus operator would behave as per expectation −
Example
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
When the above code is executed, it produces the following result −
Vector(7,8)
Data Hiding
An object's attributes may or may not be visible outside the class definition. You need to name attributes with a double underscore prefix, and those attributes then are not be directly visible to outsiders.
Comments
Post a Comment