Attributes (OOP) (OCR A-Level Computer Science): Revision Notes

Attributes (OOP)

Overview

In Object-Oriented Programming (OOP), attributes are variables that store data specific to each instance of a class. They represent the state of an object, holding values that define its unique characteristics. Attributes can be either public or private, which affects how they can be accessed and modified. Understanding attributes is essential for implementing data security, modularity, and flexibility in OOP.

Types of Attributes

Public Attributes

• Definition: Public attributes can be accessed and modified directly from outside the class. They are often used for values that do not need any control over access or modification.
• Accessing: These attributes are accessible using dot notation on an object of the class.
• Drawbacks: Public attributes can lead to unintended modifications, as they can be changed from outside the class without restrictions.

class Car:
    def __init__(self, colour):
        self.colour = colour  # Public attribute

car1 = Car("red")
print(car1.colour)  # Outputs: red
car1.colour = "blue"  # Directly modify the colour

Private Attributes

• Definition: Private attributes are restricted and can only be accessed or modified from within the class itself. In Python, private attributes are usually indicated by a single underscore (_) or a double underscore (__) before the attribute name.
• Encapsulation: Private attributes support the principle of encapsulation, which restricts direct access to an object's data. Encapsulation protects the data by controlling how it is accessed and modified.

class BankAccount:
    def __init__(self, balance):
        self.__balance = balance  # Private attribute

account = BankAccount(100)
# print(account.__balance)  # Error: Attribute is not directly accessible

Using Get and Set Methods (Accessors and Mutators)

Since private attributes cannot be accessed directly from outside the class, getter and setter methods (also known as accessors and mutators) are used to control access and modification.

Getter Methods (Accessors)

Purpose: Getter methods allow external code to access the value of a private attribute without directly exposing it. This enables controlled and secure data retrieval.

class BankAccount:
    def __init__(self, balance):
        self.__balance = balance

    def get_balance(self):
        return self.__balance  # Accessor method to retrieve balance

account = BankAccount(100)
print(account.get_balance())  # Outputs: 100

Setter Methods (Mutators)

Purpose: Setter methods allow external code to modify the value of a private attribute, with any necessary validation or restrictions applied within the method. This helps enforce data integrity.

class BankAccount:
    def __init__(self, balance):
        self.__balance = balance

    def set_balance(self, amount):
        if amount >= 0:  # Only allow non-negative balance
            self.__balance = amount
        else:
            print("Invalid amount: Balance cannot be negative.")

account = BankAccount(100)
account.set_balance(200)       # Sets balance to 200
print(account.get_balance())    # Outputs: 200
account.set_balance(-50)        # Error: Invalid amount

Attributes Example

class BankAccount:
    def __init__(self, account_holder, balance=0):
        self.account_holder = account_holder   # Public attribute
        self.__balance = balance               # Private attribute

    # Getter for balance
    def get_balance(self):
        return self.__balance

    # Setter for balance with validation
    def set_balance(self, amount):
        if amount >= 0:
            self.__balance = amount
        else:
            print("Error: Balance cannot be negative.")

    # Additional method to deposit funds
    def deposit(self, amount):
        if amount > 0:
            self.__balance += amount
            print(f"${amount} deposited. New balance: ${self.__balance}")
        else:
            print("Error: Deposit amount must be positive.")

Benefits of Using Private Attributes and Get/Set Methods

• Data Security: Private attributes ensure data is not modified unintentionally or inappropriately, supporting secure data handling.
• Data Integrity: Setter methods can validate data before modifying attributes, and maintaining correct values.
• Encapsulation: Getters and setters protect and control access to sensitive data, keeping the internal representation hidden from outside code.

Note Summary