Getter and setter methods in Python

Introduction

Getter and setter methods let you safely read and change values inside an object. They help keep data correct and private.

When you want to control how a value is read or changed in an object.

When you want to check or change data before saving it.

When you want to hide the details of how data is stored inside an object.

When you want to add extra actions when a value changes, like updating another value.

When you want to prevent direct access to important data to avoid mistakes.

Syntax

Python

class ClassName:
    def __init__(self, value):
        self._attribute = value

    def get_attribute(self):
        return self._attribute

    def set_attribute(self, new_value):
        self._attribute = new_value

Use a single underscore (_) before attribute names to show they are 'private' by convention.

Getter methods start with get_ and setter methods start with set_.

Examples

Simple getter and setter for a person's name.

Python

class Person:
    def __init__(self, name):
        self._name = name

    def get_name(self):
        return self._name

    def set_name(self, new_name):
        self._name = new_name

Setter checks if temperature is above absolute zero before setting.

Python

class Temperature:
    def __init__(self, celsius):
        self._celsius = celsius

    def get_celsius(self):
        return self._celsius

    def set_celsius(self, temp):
        if temp < -273.15:
            print('Temperature too low!')
        else:
            self._celsius = temp

Sample Program

This program creates a bank account with a balance. It uses getter and setter to read and update the balance safely. It prevents setting a negative balance.

Python

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

    def get_balance(self):
        return self._balance

    def set_balance(self, amount):
        if amount < 0:
            print('Cannot set negative balance!')
        else:
            self._balance = amount

account = BankAccount(100)
print('Initial balance:', account.get_balance())
account.set_balance(150)
print('Updated balance:', account.get_balance())
account.set_balance(-50)

Important Notes

Getter and setter methods help protect data inside objects.

Python also has a built-in @property decorator to create getters and setters more easily.

Summary

Getter methods read values safely from an object.

Setter methods update values with checks or extra steps.

They help keep data private and correct inside objects.

Practice

1. What is the main purpose of getter and setter methods in a Python class?

easy

A. To create new classes dynamically

B. To control access to private attributes safely

C. To execute code asynchronously

D. To delete objects from memory

Solution

1. Step 1: Understand getter and setter roles

   Getter methods retrieve attribute values, and setter methods update them while controlling access.

2. Step 2: Identify their purpose in encapsulation

   They protect private data by allowing controlled reading and writing, preventing direct access.

3. Final Answer:

   To control access to private attributes safely -> Option B

4. Quick Check:

   Getter/setter = control private data [OK]

Hint: Getters and setters manage private data access [OK]

Common Mistakes:

• Thinking they create new classes
• Confusing with asynchronous code
• Assuming they delete objects

2. Which of the following is the correct syntax to define a setter method for attribute age using the @property decorator in Python?

easy

A. @property.setter\ndef set_age(self, value):\n self._age = value

B. @setter.age\ndef age(self, value):\n self._age = value

C. @age.setter\ndef age(self, value):\n self._age = value

D. @age.setter\ndef set_age(self):\n self._age = value

Solution

1. Step 1: Recall setter syntax with @property

   The setter uses @attribute.setter decorator and method name matches the attribute.

2. Step 2: Check method signature

   Setter method takes self and value parameters to set the attribute.

3. Final Answer:

   @age.setter\ndef age(self, value):\n self._age = value -> Option C

4. Quick Check:

   Setter uses @age.setter and method age(self, value) [OK]

Hint: Setter uses @attribute.setter and method named attribute [OK]

Common Mistakes:

• Using wrong decorator like @setter.age
• Method name not matching attribute
• Setter missing value parameter

3. What will be the output of the following code?

class Person:
    def __init__(self, name):
        self._name = name

    @property
    def name(self):
        return self._name

    @name.setter
    def name(self, value):
        self._name = value.upper()

p = Person('alice')
p.name = 'bob'
print(p.name)

medium

A. bob

B. Error

C. alice

D. BOB

Solution

1. Step 1: Understand setter behavior

   Setter converts the assigned value to uppercase before storing it.

2. Step 2: Trace code execution

   Initially name is 'alice', then set to 'bob' which setter changes to 'BOB'. Printing returns 'BOB'.

3. Final Answer:

   BOB -> Option D

4. Quick Check:

   Setter uppercases value, output = BOB [OK]

Hint: Setter modifies value before storing, output reflects change [OK]

Common Mistakes:

• Expecting original lowercase 'bob'
• Thinking print shows initial 'alice'
• Assuming code raises error

4. Identify the error in this code snippet using getter and setter methods:

class Car:
    def __init__(self):
        self._speed = 0

    @property
    def speed(self):
        return self._speed

    @speed.setter
    def speed(self):
        self._speed = 100

c = Car()
c.speed = 50
print(c.speed)

medium

A. Setter method missing value parameter

B. Getter method missing return statement

C. Property decorator used incorrectly

D. No error, code runs fine

Solution

1. Step 1: Check setter method signature

   Setter must accept two parameters: self and value to set the attribute.

2. Step 2: Identify missing parameter

   Current setter only has self, missing value parameter, causing error on assignment.

3. Final Answer:

   Setter method missing value parameter -> Option A

4. Quick Check:

   Setter needs (self, value) parameters [OK]

Hint: Setter must have value parameter besides self [OK]

Common Mistakes:

• Omitting value parameter in setter
• Confusing getter and setter decorators
• Assuming code runs without error

5. You want to create a class Temperature that stores temperature in Celsius internally but allows getting and setting the temperature in Fahrenheit using getter and setter methods. Which code correctly implements this behavior?

hard

A. class Temperature:
def __init__(self, celsius=0):
self._celsius = celsius

@property
def fahrenheit(self):
return (self._celsius * 9/5) + 32

@fahrenheit.setter
def fahrenheit(self, value):
self._celsius = (value - 32) * 5/9

B. class Temperature:
def __init__(self, fahrenheit=32):
self._fahrenheit = fahrenheit

@property
def celsius(self):
return (self._fahrenheit - 32) * 5/9

@celsius.setter
def celsius(self, value):
self._fahrenheit = (value * 9/5) + 32

C. class Temperature:
def __init__(self, celsius=0):
self.celsius = celsius

@property
def fahrenheit(self):
return (self.celsius * 9/5) + 32

@fahrenheit.setter
def fahrenheit(self, value):
self.celsius = (value - 32) * 5/9

D. class Temperature:
def __init__(self, fahrenheit=32):
self.fahrenheit = fahrenheit

@property
def celsius(self):
return (self.fahrenheit - 32) * 5/9

@celsius.setter
def celsius(self, value):
self.fahrenheit = (value * 9/5) + 32

Solution

1. Step 1: Understand internal storage and interface

   The class stores temperature internally in Celsius (_celsius) but exposes Fahrenheit via getter and setter.

2. Step 2: Check getter and setter calculations

   Getter converts Celsius to Fahrenheit; setter converts Fahrenheit to Celsius and stores it.

3. Step 3: Verify correct use of private attribute and decorators

   class Temperature: def __init__(self, celsius=0): self._celsius = celsius @property def fahrenheit(self): return (self._celsius * 9/5) + 32 @fahrenheit.setter def fahrenheit(self, value): self._celsius = (value - 32) * 5/9 uses _celsius internally and @property/@fahrenheit.setter correctly.

4. Final Answer:

   Option A code correctly implements the behavior -> Option A

5. Quick Check:

   Internal Celsius, getter/setter convert Fahrenheit [OK]

Hint: Store Celsius internally, convert in getter/setter for Fahrenheit [OK]

Common Mistakes:

• Storing Fahrenheit internally instead of Celsius
• Using public attributes without underscore
• Mixing getter/setter names and attributes