Python Academy · Lesson

The @property Decorator

Use property for getters, setters, and deleters.

Lesson 3 of 414 steps

The @property Decorator is a free Python Academy lesson on CoddyKit. This is lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Python Academy learning path, and your progress syncs across the web and the CoddyKit app. The Python Academy course includes 4 lessons in total.

Introduction

@property turns a method into a readable attribute. @setter and @deleter complete the controlled access pattern.

Why @property?

Instead of getX()/setX() Java-style, Python uses @property. Access looks like an attribute: obj.value, not obj.get_value().

class Circle:
    def __init__(self, r): self._r = r
    @property
    def radius(self): return self._r
c = Circle(5)
print(c.radius)  # no ()

Computed Properties

@property methods compute on-the-fly. obj.area calculates every time it is accessed — no stored value needed.

import math
class Circle:
    def __init__(self, r): self._r = r
    @property
    def area(self): return math.pi * self._r**2
c = Circle(5)
print(round(c.area, 2))

Property Setter

@property_name.setter adds a setter. Now c.radius = 10 calls the setter method.

class Circle:
    def __init__(self, r): self._r = r
    @property
    def radius(self): return self._r
    @radius.setter
    def radius(self, val):
        if val <= 0: raise ValueError('radius must be positive')
        self._r = val
c = Circle(5)
c.radius = 10
print(c.radius)

Property Deleter

@property_name.deleter handles del obj.attr. Use for cleanup logic.

class User:
    def __init__(self, name): self._name = name
    @property
    def name(self): return self._name
    @name.deleter
    def name(self): del self._name
u = User('Alice')
del u.name
print(hasattr(u, '_name'))

Read-Only Property

Define only the getter (no setter). Attempting to assign raises AttributeError.

class Const:
    def __init__(self, v): self._v = v
    @property
    def value(self): return self._v
c = Const(42)
print(c.value)
try: c.value = 99
except AttributeError as e: print(e)

Property with Validation

Setters are the perfect place to validate data — check type, range, or format before storing.

class BoundedInt:
    def __init__(self, lo, hi): self.lo,self.hi=lo,hi; self._v=lo
    @property
    def value(self): return self._v
    @value.setter
    def value(self, v):
        if not self.lo <= v <= self.hi:
            raise ValueError(f'{v} out of [{self.lo},{self.hi}]')
        self._v = v
b = BoundedInt(0, 10)
b.value = 5
print(b.value)

Caching with property

Combine @property with a cached value to compute once and memoize.

class Expensive:
    _cache = None
    @property
    def result(self):
        if self._cache is None:
            self._cache = sum(range(1000))
        return self._cache
e = Expensive()
print(e.result)
print(e.result)

cached_property (Python 3.8+)

@functools.cached_property computes the property once and stores it as an instance attribute — no repeated computation.

from functools import cached_property
import math
class Circle:
    def __init__(self, r): self.r = r
    @cached_property
    def area(self): return math.pi * self.r**2
c = Circle(5)
print(round(c.area, 2))

Property vs slots

__slots__ prevents arbitrary attribute creation and saves memory. Properties add logic. They complement each other.

class Point:
    __slots__ = ('_x', '_y')
    def __init__(self, x, y): self._x,self._y=x,y
    @property
    def x(self): return self._x
p = Point(1, 2)
print(p.x)

When to Use @property

Use @property when: you need validation, computed values, or to maintain backward compatibility while changing internals.

# Before: class Person:
#     def __init__(self, age): self.age = age
# After: add validation without changing caller code
class Person:
    def __init__(self, age): self.age = age
    @property
    def age(self): return self._age
    @age.setter
    def age(self, v):
        if v < 0: raise ValueError('negative age')
        self._age = v
p = Person(30)
print(p.age)

Quick Check

What happens when you try to assign to a read-only @property (no setter defined)?

Answer it in the interactive lesson

Recap

@property: getter becomes attribute access. @name.setter adds write support. @name.deleter adds del support. @cached_property computes once. Setters are for validation.

Keep Going

Excellent progress! Keep going to master the next concept.

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Frequently Asked Questions

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Yes — the full text of “The @property Decorator” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Python Academy course, upgrade to CoddyKit PRO. The Python Academy course includes 4 lessons in total.

What will I learn in “The @property Decorator”?

Use property for getters, setters, and deleters. You practise Python Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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No prior experience is required. Python Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

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