STDIO Ghana - Python OOP

Python OOP

Seifu, Jonathon

Activity

Make lists of main dishes, side dishes, and drinks. (beef, pork, goat, sheep, chicken, fish); (akple, kenkey/dokonu, fufu, omo tuo); (water, juice, soda).

Why Object Oriented Programming

Common data and functions (inheritance)
Protect data (encapsulation)

Overview of OOP Terminology

Reference tutorialspoint

Class:A user-defined prototype for an object.
Inheritance:The creation of an instance of a class.
Instance:An individual object of a certain class.

Creating Classes

The class syntax will create a new class. The name of the class is preceeded immediately by the keyword class

class ClassName:
	#Optional class documentation string
	class_suite

Employee Class Example

class Employee:
   #Common base class for all employees
   empCount = 0 #this is a class variable accessed by Employee.empCount

   def __init__(self, name, salary):
      #this is the class constructor/initialization method called
      #when an instance is created
      self.name = name
      self.salary = salary
      Employee.empCount += 1

   def displayCount(self):
     #all methods start with self as an argument;however, when
     #you call a method python adds self to your list
     print "Total Employee %d" % Employee.empCount

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

Creating an object instance

Simply create instances of a class by calling the class name and passing whatever arguements __init__ accepts

#Create an Employee named Carl with a salary of 20
emp1 = Employee("Carl",20)

Using a class

You can invoke an internal method of an instantiated object using the dot operator

#This executes print Name : , self.name,  , Salary: , self.salary
emp1.displayEmployee()
print "total Employee %d" % Employee.empCount

Going beyond the basics

Adding an attribute e.g. emp1.age = 7
Modifying an attribute e.g. emp1.age = 8
Deleting an attribute e.g. del emp1.age

Can also use getattr(obj.name[, default]) to acess the attribute
hasattr(obj,name) to check if an attribute exists
setattr(obj,name,value) to set an attribute. This creates an attribute if it doesnt exist
delattr(obj,name) to delete an attribute.

Python Built-Ins

Python builds in

__dict__ dictionary containing class namespace
__doc__ documentation string
__module__ name in which class is defined

Example

Inheritance and Subclassing

In many programs it will be useful to have parent classes that can be used in child classes. The new, child class will inherit the methods and attributes of its parents class. Additionally, the child may include overrides. Let's look at an example.

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

Derived classes are written similar to their base classes

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

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

class Cook(Employee):
    'Common base for all cooks'
    cookCount

    def __init__(self,name,salary,isWorking, specialty):
        Employee.__init__(self,name, salary,isWorking)
        self.specialty=specialty
        Cook.cookCount += 1

    def displayCook(self):
        print "Name: ", self.name, ", Salary: ", self.salary, ", Is Working: ", self.isWorking, ", Specialty: ", self.specialty

cook1 = Cook("bob", "salary", False, "specialty")
cook1.displayCook()
emp1 = Employee(“emp1”, “salary”, False)
emp1.displayCook()

Overriding methods can be useful

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 #This prints 'Vector(7,8)'

Override Example

Write an override that compares two Vectors to see if they are the same by comparing the coordinates.