Property decorator usage in Python - Mini Project: Build & Apply

Using the Property Decorator in Python

📖 Scenario: You are creating a simple program to manage a bank account. You want to control how the account balance is accessed and updated safely.

🎯 Goal: Build a Python class called BankAccount that uses the @property decorator to get and set the account balance with validation.

📋 What You'll Learn

Create a class named BankAccount with a private attribute _balance.

Add a @property method called balance to get the current balance.

Add a setter for balance that only allows setting a non-negative value.

Print the balance after setting it.

💡 Why This Matters

🌍 Real World

Property decorators help control access to important data in programs, like bank balances, ensuring data stays valid.

💼 Career

Understanding property decorators is useful for writing clean, safe code in many Python jobs, especially in finance, data management, and software development.

1

Create the BankAccount class with a private balance

Create a class called BankAccount with an __init__ method that sets a private attribute _balance to 0.

Python

class BankAccount:
    def __init__(self):
        # Your code here

Hint

Use self._balance = 0 inside the __init__ method to create a private balance.

2

Add a property to get the balance

Add a @property method called balance inside the BankAccount class that returns self._balance.

Python

class BankAccount:
    def __init__(self):
        self._balance = 0

    # Your code here

Hint

Use @property above the balance method to make it a getter.

3

Add a setter to update the balance safely

Add a setter for balance using @balance.setter that sets self._balance only if the new value is not negative. If the value is negative, do not change self._balance.

Python

class BankAccount:
    def __init__(self):
        self._balance = 0

    @property
    def balance(self):
        return self._balance

    # Your code here

Hint

Use @balance.setter to create the setter method. Check if value is not negative before setting.

4

Create an account, set balance, and print it

Create an instance of BankAccount called account. Set account.balance to 100. Then print account.balance.

Python

class BankAccount:
    def __init__(self):
        self._balance = 0

    @property
    def balance(self):
        return self._balance

    @balance.setter
    def balance(self, value):
        if value >= 0:
            self._balance = value

# Your code here

Hint

Create the object, set the balance to 100, then print the balance to see the result.

Practice

1.

What does the @property decorator do in a Python class?

easy

A. It converts a function into a static method.

B. It makes a method private.

C. It allows a method to be accessed like an attribute.

D. It deletes an attribute from the class.

Solution

1. Step 1: Understand the role of @property

   The @property decorator lets you call a method without parentheses, like an attribute.

2. Step 2: Compare options

   Only It allows a method to be accessed like an attribute. correctly describes this behavior. Other options describe unrelated features.

3. Final Answer:

   It allows a method to be accessed like an attribute. -> Option C

4. Quick Check:

   @property makes method act like attribute [OK]

Hint: Remember: @property hides () making method look like attribute [OK]

Common Mistakes:

• Thinking @property makes method private
• Confusing @property with @staticmethod
• Believing @property deletes attributes

2.

Which of the following is the correct syntax to define a setter for a property named value?

class MyClass:
    @property
    def value(self):
        return self._value

    # What goes here?

easy

A. @setter.value\ndef value(self, val):\n self._value = val

B. @value.setter\ndef value(self, val):\n self._value = val

C. @value.set\ndef set_value(self, val):\n self._value = val

D. @value.setter\ndef set_value(self, val):\n self._value = val

Solution

1. Step 1: Identify correct setter syntax

   The setter uses the property name with @value.setter and defines a method with the same name value.

2. Step 2: Check method name and decorator

   @value.setter\ndef value(self, val):\n self._value = val correctly uses @value.setter and method value. Others use wrong decorator or method names.

3. Final Answer:

   @value.setter\ndef value(self, val):\n self._value = val -> Option B

4. Quick Check:

   Setter uses @propertyname.setter and same method name [OK]

Hint: Setter decorator is @propertyname.setter with same method name [OK]

Common Mistakes:

• Using @setter.value instead of @value.setter
• Changing method name in setter
• Using @value.set instead of @value.setter

3.

What will be the output of the following code?

class Circle:
    def __init__(self, radius):
        self._radius = radius

    @property
    def radius(self):
        return self._radius

    @radius.setter
    def radius(self, value):
        if value < 0:
            self._radius = 0
        else:
            self._radius = value

c = Circle(5)
c.radius = -3
print(c.radius)

medium

A. 0

B. 5

C. -3

D. AttributeError

Solution

1. Step 1: Understand setter logic

   When setting radius, if value < 0, it sets _radius to 0, else to value.

2. Step 2: Trace code execution

   Initial radius is 5. Then c.radius = -3 triggers setter, sets _radius to 0. Printing c.radius returns 0.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Setter sets negative radius to 0 [OK]

Hint: Setter changes negative radius to zero, so output is 0 [OK]

Common Mistakes:

• Expecting original value 5 to remain
• Printing -3 instead of 0
• Confusing property with direct attribute

4.

Find the error in this code using property decorators and fix it.

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

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

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

p = Person('Alice')
p.name = 'Bob'
print(p.name)

medium

A. Change setter method name to name instead of set_name.

B. Remove the @property decorator.

C. Change self._name to self.name in setter.

D. Add a deleter method for name.

Solution

1. Step 1: Identify setter method name mismatch

   The setter decorator @name.setter requires the method to be named name, but here it is set_name.

2. Step 2: Fix method name

   Rename the setter method to name to match the property name and decorator.

3. Final Answer:

   Change setter method name to name instead of set_name. -> Option A

4. Quick Check:

   Setter method name must match property name [OK]

Hint: Setter method name must match property name exactly [OK]

Common Mistakes:

• Using different method name for setter
• Removing @property decorator mistakenly
• Changing attribute name inside setter

5.

Consider a class that stores a temperature in Celsius internally but exposes it as Fahrenheit using property decorators. Which code correctly implements this?

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

    @property
    def fahrenheit(self):
        # Convert Celsius to Fahrenheit
        return (self._celsius * 9/5) + 32

    @fahrenheit.setter
    def fahrenheit(self, value):
        # Convert Fahrenheit to Celsius
        self._celsius = (value - 32) * 5/9

# Usage
temp = Temperature(0)
temp.fahrenheit = 212
print(round(temp._celsius))

What is the output?

hard

A. 32

B. 212

C. 0

D. 100

Solution

1. Step 1: Understand property getter and setter

   The getter converts Celsius to Fahrenheit. The setter converts Fahrenheit back to Celsius and stores it.

2. Step 2: Trace the code

   Initially, Celsius is 0. Setting temp.fahrenheit = 212 calls setter, converts 212°F to Celsius: (212-32)*5/9 = 100. Printing temp._celsius rounded gives 100.

3. Final Answer:

   100 -> Option D

4. Quick Check:

   Setter converts Fahrenheit to Celsius correctly [OK]

Hint: Setter converts Fahrenheit to Celsius, so 212°F = 100°C [OK]

Common Mistakes:

• Confusing getter and setter conversions
• Printing Fahrenheit instead of Celsius
• Not rounding the output