Understanding Classes and Objects in Python
Introduction to classes in Python: defining classes, creating objects, defining attributes and methods, class instantiation with __init__ method. Learn how to work with classes and objects in Python effectively.
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Objectives O This chapter introduces 1. Introduction to a class Class design Class object Method object Class and instance variables 2. 3. 4. 5. 2
Classes O Classes provide a means of bundling data and functionality together O Creating a new class creates a new type of object, allowing new instances of that type to be made O Each class instance can have attributes attached to it for maintaining its state O Class instances can also have methods (defined by its class) for modifying its state 3
Class Definition O Class definitions, like function definitions (def statements) must be executed before they have any effect O The statements inside a class definition will usually be function definitions, but other statements are allowed O When a class definition is left normally (via the end), a class object is created class ClassName: <statement-1> <statement-N> 4
Class Objects O Class objects support two kinds of operations: attribute references and instantiation O Attribute references use the standard syntax used for all attribute references in Python: obj.name obj.name class MyClass: i = 12345 def f(self): return hello world O MyClass.i and MyClass.f are valid attribute references, returning an integer and a function object 5
Class Objects O Class instantiation uses function notation O Just pretend that the class object is a parameterless function that returns a new instance of the class class MyClass: i = 12345 def f(self): #It will call Myclass.f(obj) return hello world obj = MyClass() O The above code creates a new instance of the class and assigns this object to the variable obj obj 6
Class Objects O Many classes like to create objects with instances customized to a specific initial state O A class may define a special method named __init__() O When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly created class instance class MyClass: def __init__(self): #It will call Myclass.__init__(obj) print('hello') 7 obj = MyClass() # hello will be printed when the class instance is created
Method Objects objfunc = obj.f while True: print(objfunc()) #It will continue to print hello world until the end of time class MyClass: i = 12345 def f(self): return hello world obj = MyClass() O In the MyClass example, obj.f the string 'hello world O However, it is not necessary to call a method right away O obj.f is a method object, which can be stored away and called at a later time obj.f() () will return 8
Method Objects objfunc = obj.f while True: print(objfunc()) #It will continue to print hello world until the end of time class MyClass: i = 12345 def f(self): return hello world obj = MyClass() O A method object is created by packing the instance object and the function object just found together in an abstract object O When the method object is called with an argument list, a new argument list is constructed from the instance instance object object and and the the argument function object is called with this new argument list O obj.f() is equivalent to MyClass.f(x) the argument list list, and the 9
Class and Instance Variables O Instance variables Instance variables are for data unique to each instance O Class variables Class variables are for attributes and methods shared by all instances of the class shared by all instances of the class class Dog: kind = canine # class variable def __init__(self, name): self.name = name # instance variable class variable shared by all instances instance variable unique to each instance d = Dog('Fido') e = Dog('Buddy') 10
Class and Instance Variables class Dog: kind = canine # class variable def __init__(self, name): self.name = name # instance variable class variable shared by all instances instance variable unique to each instance d = Dog('Fido') e = Dog('Buddy') O d.kind and e.kind have the same string 'canine' O Because class variable is shared by all instances O d.name and e.name are 'Fido and 'Buddy , respectively O Because instance variable unique to each instance11
Class and Instance Variables class Dog: kind = canine # class variable def __init__(self, name): self.name = name # instance variable class variable shared by all instances instance variable unique to each instance d = Dog('Fido') e = Dog('Buddy') O Instance variables need not be declared O Like local variables O They spring into existence when they are first assigned to O For example d.age = 15 print(d.age) del d.age 12
Examples class MyClass: def __init__(self, num): self.attr1 = num obj1 = MyClass(10) obj2 = MyClass(20) print(obj1.attr1) #output 10 print(obj2.attr1) #output 20 13
Examples class MyClass: attr2 = 5 def __init__(self, num): self.attr1 = num MyClass.attr2 += 1 #Use classname to access class variables obj1 = MyClass(10) print(obj1.attr2) #output 6 obj2 = MyClass(20) print(obj1.attr2) #output 7 print(obj2.attr2) #output 7 print(MyClass.attr2) #output 7 14
Examples class Person: count = 0 def __init__(self, name, height): self.name = name self.height = height Person.count += 1 users = {} users['Andy'] = Person('Andy', 150) print(users['Andy'].name, users['Andy'].height, users['Andy'].count) users['Bob'] = Person('Bob', 176) users['Mary'] = Person('Mary', 180) users['Andy'].height += 20 users['Bob'].name = 'John' for i in sorted(users.keys()): print(users[i].name, users[i].height, users[i].count) 15 15
Default Methods O The classes in python provide several default methods O __str__() O The __str__ method in Python represents the class objects as a string class Person: def __init__(self, name , age): self.name = name self.age = age def __str__(self): return "Hi! My name is " + self.name + ".\nI'm " + str(self.age) + " years old." Output: 16 person1 = Person('John', 24) print(person1)
Default Methods O __del__() O The __del__() method is referred to as a destructor method O It is called after collection occurs, which happens after all references to the item have been destroyed. an object s garbage class Person: def __init__(self, name , age): self.name = name self.age = age print(self.name + ': Hi!') def __del__(self): print(self.name + ': Goodbye!') person1 = Person('John', 24) del person1 17 Output:
Exercise O Please design a program that enables the following instructions: O Input 0: Exit program O Input 1: Insert student data, including ID, name, and score O Input 2: Delete student data by the specified ID O Input 3: Update student data by the specified ID O Input 4: Show all student information 18
Source O References: O https://docs.python.org/3/tutorial/classes.ht ml 19
