Property decorator usage in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the @property decorator in Python?

The @property decorator allows you to define a method that can be accessed like an attribute. It helps to control access to instance variables while keeping the syntax simple and clean.

intermediate

How do you define a setter method for a property in Python?

You define a setter method by using the @property_name.setter decorator above a method with the same name as the property. This method allows you to set the value while controlling or validating it.

beginner

What happens if you try to set a value to a property without defining a setter?

If no setter is defined, trying to assign a value to the property will raise an AttributeError. This protects the property from being changed directly.

beginner

Explain the difference between a regular method and a property method in Python.

A regular method requires parentheses to be called (e.g., obj.method()), while a property method can be accessed like an attribute without parentheses (e.g., obj.property). Properties provide a way to use methods as if they were attributes.

beginner

Show a simple example of a class with a property and a setter using the <code>@property</code> decorator.

Example:<br><pre>class Person:
    def __init__(self, name):
        self._name = name

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

    @name.setter
    def name(self, value):
        if not value:
            raise ValueError('Name cannot be empty')
        self._name = value

p = Person('Alice')
print(p.name)  # Outputs: Alice
p.name = 'Bob'  # Sets new name
</pre>

What does the @property decorator do in Python?

A Turns a method into a read-only attribute

B Defines a class method

C Creates a static method

D Deletes an attribute

The @property decorator makes a method behave like a read-only attribute.

How do you define a setter for a property named age?

A Using <code>@age.set</code>

B Using <code>@property.setter</code>

C Using <code>@set_age</code>

D Using <code>@age.setter</code> above the setter method

The setter for a property age is defined with @age.setter.

What error occurs if you try to set a property without a setter?

A NameError

B AttributeError

C ValueError

D TypeError

Trying to set a property without a setter raises an AttributeError.

Which of these is a benefit of using properties?

A Control access to attributes with simple syntax

B Make methods run faster

C Allow multiple inheritance

D Automatically document code

Properties let you control attribute access while keeping syntax clean.

How do you access a property method in Python?

A Using square brackets

B With parentheses like a normal method

C Like an attribute, without parentheses

D Using the <code>get()</code> function

Property methods are accessed like attributes, without parentheses.

Explain how the @property decorator works and why it is useful.

Think about how you can hide internal details but still let users get or set values easily.

Describe how to create a property with both getter and setter methods in a Python class.

Remember the setter decorator uses the property name.

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