Movatterモバイル変換

http://www.skillbrew.com/SkillbrewTalent brewed by theindustry itselfClasses and ObjectsPavan Verma@YinYangPavanFounder, P3 InfoTech Solutions Pvt. Ltd.1Python Programming Essentials

© SkillBrew http://skillbrew.comContents Defining a class Class attributes Class methods Class instances __init__ method self keyword Accessing attributes and methods Deleting attributes Types of attributes Inheritance Method overriding Calling parent functions2

© SkillBrew http://skillbrew.comDefining a classclass Calculator():counter = 0def __init__(self):passdef add(self):passclass keyword todefine a classA class definition creates a class object from whichclass instances may be created4

© SkillBrew http://skillbrew.comClass Instancescalc = Calculator()• In order to you use it we create an instance ofclass• Instances are objects created that use the classdefinitionJust call the class definition like a function to createa class instance7

© SkillBrew http://skillbrew.com__init__ method• __init__ method is like an initializationconstructor• When a class defines an __init__ ()method, class instantiation automatically invokes__init__() method for the newly createdclass instance8

© SkillBrew http://skillbrew.com__init__ method (2)class Calculator():counter = 0def __init__(self, x=0, y=0):self.x = xself.y = ydef add(self):passcalc = Calculator(10, 20)print calc.xprint calc.yOutput:10209

© SkillBrew http://skillbrew.comself keyword• The first argument of every method is a referenceto the current instance of the class• By convention, we name this argument self• In __init__, self refers to the object currentlybeing created• In other class methods, it refers to the instancewhose method was called• Similar to the keyword this in Java or C++10

© SkillBrew http://skillbrew.comAccessing attributes and methodsUse the dot operator to access class attributes andmethodscalc = Calculator(10, 20)print calc.xprint calc.yprint calc.counterOutput:1020011

© SkillBrew http://skillbrew.comAccessing attributes and methods (2)class Calculator():counter = 0def __init__(self, x=0, y=0):self.x = xself.y = ydef add(self):return self.x + self.ycalc = Calculator(10, 20)print calc.add()• Although you must specify selfexplicitly when defining themethod, you don’t include itwhen calling the method• Python passes it for youautomatically12

© SkillBrew http://skillbrew.comDeleting Instances• When you are done with an object , you don’t haveto delete or free it explicitly• Python has automatic garbage collection• Python will automatically detect when all referencesto a piece of memory have gone out of scope.Automatically frees the memory.• Garbage collection works well, hence fewer memoryleaks• There’s also no “destructor” method for classes.13

© SkillBrew http://skillbrew.comData attributesclass Calculator():counter = 0def __init__(self, x=0, y=0):self.x = xself.y = ydef add(self):return self.x + self.ycalc = Calculator(10, 20)print calc.x # 10print calc.y # 20calc2 = Calculator(15, 35)print calc2.x # 15print calc2.y # 35• Data attributes arevariables owned by aparticular instance• Each instance has its ownvalue for data attributes16

© SkillBrew http://skillbrew.comData attributes (2)class Calculator():counter = 0def __init__(self, x=0, y=0):self.x = xself.y = ycalc = Calculator(10, 20)calc.z = calc.x + calc.yprint calc.zprint calc.__dict__Output:30{'y': 20, 'x': 10, 'z': 30}17• In Python classes you don’thave a restriction ofdeclaring all dataattributes before hand,you can create dataattributes at runtimeanywhere• calc.z is an attributewhich is defined atruntime outside the classdefinition

© SkillBrew http://skillbrew.comClass attributesclass Calculator():counter = 0def __init__(self, x=0, y=0):self.x = xself.y = ydef add(self):self.__class__.counter += 1return self.x + self.ycalc = Calculator(10, 20)print calc.add()print calc.counter # 1calc2 = Calculator(30, 40)print calc2.add()print calc2.counter # 2• Access the classattribute usingself.__class__.counter• Class attributes areshared among allinstances18

© SkillBrew http://skillbrew.comClass attributes (2) Class attributes are defined within a class definitionand outside of any method Because all instances of a class share one copy of aclass attribute, when any instance changes it, thevalue is changed for all instancesself.__class__.attribute_name19

© SkillBrew http://skillbrew.comData attributes Variable owned by aparticular instance Each instance has its ownvalue for it These are the mostcommon kind of attributeClass attributes Owned by the class as awhole All class instances share thesame value for it Good for• Class-wide constants• Building counter of howmany instances of theclass have been madeData attributes vs Class attributes20

© SkillBrew http://skillbrew.comInheritanceclass Shape(object):def name(self, shape):print "Shape: %s" % shapeclass Square(Shape):def area(self, side):return side**2Shape is theparent classSquare is thechild class inheritsShapeclass Parent(object):passclass Child(Parent):pass22

© SkillBrew http://skillbrew.comInheritance (2)class Shape(object):def name(self, shape):print "Shape: %s" % shapeclass Square(Shape):def area(self, side):return side**2s = Square()s.name("square")print s.area(2)Output:Shape: square4Child class Square has access toParent classes methods andattributes23

© SkillBrew http://skillbrew.comMethod overridingclass Shape(object):def name(self, shape):print "Shape: %s" % shapeclass Square(Shape):def name(self, shape):print "Child class Shape %s" % shapedef area(self, side):return side**2s = Square()s.name("square")print s.area(2)Output:Child class Shape square424

© SkillBrew http://skillbrew.comCalling the parent methodclass Shape(object):def name(self, shape):print "Shape: %s" % shapeclass Square(Shape):def name(self, shape):super(Square, self).name(shape)print "Child class Shape %s" % shapedef area(self, side):return side**2s = Square()s.name("square")Use super keyword to call parent class methodsuper(ChildClass, self).method(args)25

© SkillBrew http://skillbrew.comClass & Static Methodsclass Calculator(object):counter = 0def __init__(self, x=0, y=0):...def add(self):...@classmethoddef update_counter(cls):cls.counter += 1@staticmethoddef show_help():print 'This calculator canadd'26

© SkillBrew http://skillbrew.comResources http://www.diveintopython.net/object_oriented_framework/defining_classes.html http://docs.python.org/2/tutorial/classes.html http://docs.python.org/2/library/functions.html#super27

Movatterモバイル変換

Change Language

Python Programming Essentials - M20 - Classes and Objects

Embed presentation

Recommended

More Related Content

What's hot

Viewers also liked

Similar to Python Programming Essentials - M20 - Classes and Objects

More from P3 InfoTech Solutions Pvt. Ltd.

Python Programming Essentials - M20 - Classes and Objects