Properties

Expose managed attributes with the @property decorator.

Syntax@property def area(self): ...

A property lets a method be accessed like an attribute — no parentheses. It is perfect for computed values or for validating data when it is set.

How it works

  • Decorate a method with @property to make it a read-only attribute.
  • Add a matching @name.setter to control assignment.

Example

Example · python
class Circle:
    def __init__(self, radius):
        self.radius = radius

    @property
    def area(self):
        return 3.14159 * self.radius ** 2

c = Circle(2)
print(round(c.area, 2))

# Output:
# 12.57

When to use it

  • A Temperature class exposes 'celsius' and 'fahrenheit' as properties that stay in sync automatically.
  • A User class validates and normalises an email address whenever the 'email' property is set.
  • A read-only 'full_name' property computes first + last name without allowing direct assignment.

More examples

Read-only property

Defines a computed, read-only property area that is accessed like an attribute, not a method call.

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

    @property
    def area(self):
        return 3.14159 * self._radius ** 2

c = Circle(5)
print(c.area)   # 78.53975
# c.area = 10  # AttributeError: can't set

Property with setter validation

Uses a setter to validate the value before storing it, raising an error for physically impossible temperatures.

Example · python
class Temperature:
    def __init__(self, celsius):
        self.celsius = celsius

    @property
    def celsius(self):
        return self._celsius

    @celsius.setter
    def celsius(self, value):
        if value < -273.15:
            raise ValueError('Below absolute zero')
        self._celsius = value

t = Temperature(25)
print(t.celsius)   # 25
t.celsius = 100    # OK
print(t.celsius)   # 100

Computed property pair

Provides two computed properties that derive their values from stored width and height attributes.

Example · python
class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height

    @property
    def area(self):
        return self.width * self.height

    @property
    def is_square(self):
        return self.width == self.height

r = Rectangle(4, 4)
print(r.area)       # 16
print(r.is_square)  # True

Related

py dunderpy decorators

Discussion

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