Master Python OOP: Classes, Objects, and Inheritance Explained

Classes and Objects

A class describes a collection of objects that share the same attributes and methods; an object is an instance of a class. Example: a

Student

class with

name

and

score

attributes.

Attributes and Methods

Attribute : shared property of all objects, e.g., name and score.

Method : function defined inside a class, e.g., a method to print a student's information.

Defining a Class in Python

<code>class Student():
    def __init__(self, name, score):
        self.name = name
        self.score = score
    def out(self):
        print("%s：%s" % (self.name, self.score))
</code>

Creating objects:

<code>Student1 = Student('Anny', '100')
Student2 = Student('Mike', '90')
</code>

Access attributes with

Student1.name

and call methods with

Student1.out()

.

Class Variables vs Instance Variables

A class variable is shared across all instances; an instance variable belongs to each object. Example with a counter:

<code>class Student():
    number = 0
    def __init__(self, name, score):
        self.name = name
        self.score = score
        Student.number = Student.number + 1
    def show(self):
        print("%s：%s" % (self.name, self.score))
</code>

The original code raised

UnboundLocalError

because it tried to modify

number

without qualification; the corrected version uses

Student.number

.

Class Methods

Methods that belong to the class itself use

cls

and are marked with

@classmethod

. Example prints the total number of students.

<code>class Student():
    number = 0
    def __init__(self, name, score):
        self.name = name
        self.score = score
        Student.number = Student.number + 1
    def show(self):
        print("%s：%s" % (self.name, self.score))
    @classmethod
    def people(cls):
        print("一共有%s名学生" % Student.number)
</code>

Private Attributes and Methods

Attributes and methods prefixed with double underscores are private and cannot be accessed directly from outside the class. Use the

@property

decorator to expose them safely.

<code>@property
def scores(self):
    print("该学生成绩为%s" % self.score)
</code>

Inheritance

Subclass inherits attributes and methods from a parent class, reducing code duplication. Example with a base class

SchoolMember

and subclasses

Teacher

and

Student

.

<code>class SchoolMember:
    def __init__(self, name, age):
        self.name = name
        self.age = age
    def tell(self):
        print('Name:"{}" Age:"{}"'.format(self.name, self.age), end=" ")

class Teacher(SchoolMember):
    def __init__(self, name, age, salary):
        SchoolMember.__init__(self, name, age)
        self.salary = salary
    def tell(self):
        SchoolMember.tell(self)
        print('Salary: {}'.format(self.salary))

class Student(SchoolMember):
    def __init__(self, name, age, score):
        SchoolMember.__init__(self, name, age)
        self.score = score
    def tell(self):
        SchoolMember.tell(self)
        print('score: {}'.format(self.score))
</code>

Shows how to call parent methods using the class name or

super()

.

Using super()

<code>class Student(SchoolMember):
    def __init__(self, name, age, score):
        SchoolMember.__init__(self, name, age)
        self.score = score
    def tell(self):
        super().tell()
        print('score: {}'.format(self.score))
</code>