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Read Only Properties in Python

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  • Last Updated : 10 Jul, 2020

Prerequisites: Python Classes and Objects

A class is a user-defined blueprint or prototype from which objects are created. Classes provide a means of bundling data and functionality together. Creating a new class creates a new type of object, allowing new instances of that type to be made. To put it in simple words, let us assume a class Student, a student can have many properties such as Name, Course, Student’s ID, etc. Now let us assume we have a Student named Anita pursuing MBA, Anita is an object of class Student.

Example:




class student:
      
    def __init__(self, name, course):
        self.name = name
        self.course = course
  
    def studentid(self):
        return "student's identification number is \
        {}{}".format(self.name, self.course)
  
student1 = student("Anita", "MBA")
print(student1.studentid())

Output:

student's identification number is AnitaMBA

Now, We want to access student id as a property and not as a method. For this all required is to add @property decorator before the method.

Example:




class student:
      
    def __init__(self, name, course):
        self.name = name
        self.course = course
  
    @property
    def studentid(self):
        return "student's identification number is \
        {}{}".format(self.name, self.course)
  
student1 = student("Anita", "MBA")
print(student1.studentid)

Output:

student's identification number is AnitaMBA

A Read-Only-Property is a property decorator without a setter. The student id here is a read-only-property since it doesn’t have a setter. In general terms, it means that the value is not changeable. To understand let’s take one more example:

Example:




class employee:
    def __init__(self, basesalary, yearsofworking):
        self.basesalary = basesalary
        self.yearsofworking = yearsofworking
      
    @property
    def salary(self):
        self.salary = 50000
  
amit = employee(20000, 5)
amit.salary = 10000
print(amit.basesalary, amit.yearsworking, amit.salary)

Output:

Traceback (most recent call last):
File “/home/e029e0b9ccad85905e22dd5a91943897.py”, line 14, in
amit.salary = 10000
AttributeError: can’t set attribute

To fix this one setter is to be added in this code. After doing this, it will no longer be a read-only-property.

Example:




class employee:
    def __init__(self, basesalary, yearsofworking):
        self.basesalary = basesalary
        self.yearsofworking = yearsofworking
        self._salary = 0
      
    @property
    def salary(self):
        return self._salary
  
    @salary.setter
    def salary(self, salary):
        self._salary = salary
  
amit = employee(20000, 5)
amit.salary = 10000
print(amit.basesalary, amit.yearsofworking, amit.salary)

Output:

20000 5 10000

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