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Python for Ethical Hackers
Mohammad reza Kamalifard
Kamalifard.ir/pysec101

Python Language Essentials
Object Oriented Programing
kamalifard@datasec.ir

Object
Building block of program
Component with some desired functionality
You ask objects to do work.
You don't know how they do that.

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 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.

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.
Instance variable: A variable that is defined inside a method and belongs
only to the current instance of a class.
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.

A Simple Class in 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

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.
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.
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.

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()
Name : Zara , Salary: 2000
Name : Manni , Salary: 5000
Total Employee 2

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):
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)

Add, Remove or Modify
You can add, remove or modify attributes of classes and
objects at any time:
emp1.age = 22 # Add an 'age' attribute.
emp1.age = 18 # Modify 'age' attribute.
del emp1.age # Delete 'age' attribute.

Add, Remove or Modify
Instead of using the normal statements to access attributes, you can
use following functions:
The getattr(obj, name[, default]) : to access the attribute of object.
The hasattr(obj,name) : to check if an attribute exists or not.
The setattr(obj,name,value) : to set an attribute. If attribute does not
exist, then it would be created.
The delattr(obj, name) : to delete an attribute.
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'

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.

>>>print "Employee.__doc__:", Employee.__doc__
>>>print "Employee.__name__:", Employee.__name__
>>>print "Employee.__module__:", Employee.__module__
>>>print "Employee.__dict__:", Employee.__dict__
Employee.__doc__: Common base class for all employees
Employee.__name__: Employee
Employee.__module__: __main__
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>}

Destroying Objects (Garbage Collection)
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.
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.
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.

Destroying Objects (Garbage Collection)
a = 40 # Create object <40>
b = a # Increase ref. count of <40>
c = [b] # Increase ref. count of <40>
del a # Decrease ref. count of <40>
b = 100 # Decrease ref. count of <40>
c[0] = -1 # Decrease ref. count of <40>
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.

Destructor __del__()
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 objects
del pt1
del pt2
del pt3
140586269057680 140586269057680 140586269057680
Point destroyed

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.
class SubClassName (ParentClass1[, ParentClass2, ...]):
'Optional class documentation string'
class_suite

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
Calling child constractor
Calling child method
Calling parent Method
Parent attribute : 200

Similar way, you can drive a class from multiple parent classes
as follows:
class A: # define your class A
.....
class B: # define your calss B
.....
class C(A, B): # subclass of A and B
.....
You can use issubclass() or isinstance() functions to check a relationships of
two classes and instances.
The issubclass(sub, sup) boolean function returns true if the given subclass
sub is indeed a subclass of the superclass sup.
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

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.
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
Calling child method

Base Overloading Methods
__init__ ( self [,args...] )
Constructor (with any optional arguments)
Sample Call : obj = className(args)
_del__( self )
Destructor, deletes an object
Sample Call : dell obj
__repr__( self )
Evaluatable string representation
Sample Call : repr(obj)
__str__( self )
Printable string representation
Sample Call : str(obj)
__cmp__ ( self, x )
Object comparison
Sample Call : cmp(obj, x)

Data Hiding
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.
class JustCounter:
__secretCount = 0
def count(self):
self.__secretCount += 1
print self.__secretCount
counter = JustCounter()
counter.count()
counter.count()
print counter.__secretCount

1
2
Traceback (most recent call last):
File "test.py", line 12, in <module>
print counter.__secretCount
AttributeError: JustCounter instance has no attribute '__secretCount'
Python protects those members by internally changing the name to include the class
name. You can access such attributes as object._className__attrName. If you would
replace your last line as following, then it would work for you:
.........................
print counter._JustCounter__secretCount
When the above code is executed, it produces the following result:
1
2
2

This work is licensed under the Creative Commons
Attribution-NoDerivs 3.0 Unported License.
To view a copy of this license, visit http://creativecommons.org/licenses/by-nd/3.0/
Copyright 2013 Mohammad reza Kamalifard.
All rights reserved.

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