Properties

Property Decorator

  1. # @property for getter
  2. class Circle:
  3.     def __init__(self, radius):
  4.         self._radius = radius
  5.     @property
  6.     def radius(self):
  7.         return self._radius
  8. Circle(5).radius
  9. #ans: 5 (accessed like attribute)

Property Setter

  1. # @property.setter
  2. class Circle:
  3.     def __init__(self, radius):
  4.         self._radius = radius
  5.     @property
  6.     def radius(self):
  7.         return self._radius
  8.     @radius.setter
  9.     def radius(self, value):
  10.         if value < 0:
  11.             raise ValueError("Negative")
  12.         self._radius = value

Computed Property

  1. # property with calculation
  2. class Circle:
  3.     def __init__(self, radius):
  4.         self._radius = radius
  5.     @property
  6.     def area(self):
  7.         return 3.14 * self._radius ** 2
  8. Circle(5).area
  9. #ans: 78.5

Read-Only Property

  1. # property without setter
  2. class Person:
  3.     def __init__(self, name):
  4.         self._name = name
  5.     @property
  6.     def name(self):
  7.         return self._name
  8. person = Person("Alice")
  9. #ans: person.name = "Bob"  # AttributeError

Property Deleter

  1. # @property.deleter
  2. class Person:
  3.     def __init__(self, name):
  4.         self._name = name
  5.     @property
  6.     def name(self):
  7.         return self._name
  8.     @name.deleter
  9.     def name(self):
  10.         del self._name
  11. person = Person("Alice")
  12. del person.name

Exercises - Part 1

  1. # property vs method?
  2. class MyClass:
  3.     @property
  4.     def value(self):
  5.         return 5
  6. obj = MyClass()
  7. obj.value
  8. #ans: 5 (no parentheses)

Exercises - Part 2

  1. # setter without getter?
  2. class MyClass:
  3.     @value.setter
  4.     def value(self, x):
  5.         pass
  6. #ans: NameError (@property needed first)

Exercises - Part 3

  1. # property assignment?
  2. class MyClass:
  3.     @property
  4.     def value(self):
  5.         return 5
  6. obj = MyClass()
  7. obj.value = 10
  8. #ans: AttributeError (no setter)

Exercises - Part 4

  1. # property caching?
  2. class MyClass:
  3.     @property
  4.     def value(self):
  5.         print("Computing")
  6.         return 5
  7. obj = MyClass()
  8. obj.value
  9. obj.value
  10. #ans: prints "Computing" twice

Exercises - Part 5

  1. # property validation?
  2. class MyClass:
  3.     @property
  4.     def x(self):
  5.         return self._x
  6.     @x.setter
  7.     def x(self, value):
  8.         if value < 0:
  9.             raise ValueError()
  10.         self._x = value

Exercises - Part 6

  1. # property in __init__?
  2. class MyClass:
  3.     def __init__(self, x):
  4.         self.x = x  # calls setter
  5.     @property
  6.     def x(self):
  7.         return self._x
  8.     @x.setter
  9.     def x(self, value):
  10.         self._x = value

Exercises - Part 7

  1. # delete without deleter?
  2. class MyClass:
  3.     @property
  4.     def value(self):
  5.         return 5
  6. obj = MyClass()
  7. del obj.value
  8. #ans: AttributeError

Exercises - Part 8

  1. # property inheritance?
  2. class Base:
  3.     @property
  4.     def x(self):
  5.         return 5
  6. class Derived(Base):
  7.     pass
  8. Derived().x
  9. #ans: 5

Exercises - Part 9

  1. # override property?
  2. class Base:
  3.     @property
  4.     def x(self):
  5.         return 5
  6. class Derived(Base):
  7.     @property
  8.     def x(self):
  9.         return 10
  10. Derived().x
  11. #ans: 10

Exercises - Part 10

  1. # property with args?
  2. class MyClass:
  3.     @property
  4.     def value(self, arg):
  5.         return arg
  6. #ans: TypeError (property takes no args)

Google tag (gtag.js)