Properties

Getters

Let’s create a shapes.py file and in it create a Circle class which includes attributes of a radius and an area:

import math

class Circle:
    def __init__(self, radius=1):
        self.radius = radius
        self.area = math.pi * self.radius ** 2

This works, but when we change our radius, the area doesn’t change:

>>> from shapes import Circle
>>> circle = Circle()
>>> circle.radius
1
>>> circle.area
3.141592653589793
>>> circle.radius = 5
>>> circle.area
3.141592653589793

We could make a “getter” method that returns the area:

import math

class Circle:

    def __init__(self, radius=1):
        self.radius = radius

    def get_area(self):
        return math.pi * self.radius ** 2

But that would result in a somewhat awkward interface. We would have to type circle.get_area() instead of circle.area.

Instead, let’s make this method into a property by using the property decorator. The decorator syntax, using the @ symbol, is a way to apply a “decorator function” to our own function.

Decorators are said to “wrap” a function because they can change the behavior of that function, usually adding some extra functionality before or after a function runs and sometimes entirely replacing the function.

Note

A decorator function is a function that takes a function as input and returns a new replacement function. For more on decorators, see the “Stuff we Skipped” section.

The property decorator, when applied to a method, makes the method name behave just like an attribute variable.

import math

class Circle:

    def __init__(self, radius=1):
        self.radius = radius

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

This allows us to access our area attribute as if it was just any other attribute, not a method:

>>> from shapes import Circle
>>> circle = Circle()
>>> circle.radius
1
>>> circle.area
3.141592653589793
>>> circle.radius = 5
>>> circle.area
78.53981633974483

Setters

Let’s add a diameter property to our Circle class:

import math

class Circle:

    def __init__(self, radius=1):
        self.radius = radius

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

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

Let’s try it out:

>>> from shapes import Circle
>>> circle = Circle()
>>> circle.radius
1
>>> circle.diameter
2
>>> circle.radius = 5
>>> circle.diameter
10

What if we want to set the diameter though?

>>> circle.diameter = 20
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't set attribute

We can’t change the diameter because this is a read-only property.

To allow writing to this property, we have to make a setter method for it:

import math

class Circle:

    def __init__(self, radius=1):
        self.radius = radius

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

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

    @diameter.setter
    def diameter(self, diameter):
        self.radius = diameter / 2

The syntax is a little weird for that, but it works. In that @diameter.setter line we’re telling our already created diameter property object that we’d like to use a particular function as our setter method.

Now we can both get and set our diameter. When we change either the diameter or the radius, the area will always change appropriately:

>>> from shapes import Circle
>>> circle = Circle()
>>> circle.radius
1
>>> circle.diameter
2
>>> circle.diameter = 10
>>> circle.radius
5.0
>>> circle.area
78.53981633974483
>>> circle.radius = 2
>>> circle.diameter
4
>>> circle.area
12.566370614359172

This feature of Python allows more control of the attributes of classes. For example, perhaps our Circle class is defined in a specific application where it does not make sense for the radius to be less than 1.0. We could use a property.setter decorator to check the new radius value, and raise a ValueError if it is an invalid number.

In this way, we can always make sure that our attributes contain valid data.

Property Exercises

Point with Magnitude

This is the Point exercise in properties.py.

Make a Point class that has an auto-calculated magnitude attribute. The magnitude should be calculated by sqrt(x**2 + y**2 + z**2).

Note

To test these changes, you should modify the PointTests class in properties_test.py to comment out the @unittest.skip for “magnitude property”.

Example:

>>> from properties import Point
>>> p = Point(2, 3, 6)
>>> p.magnitude
7.0
>>> p.y = 9
>>> p.magnitude
11.0

Full Name

This is the Person exercise in properties.py.

Create the Person class so that it has a first_name and last_name attribute and a name property which is the combination first_name and last_name, with a space between.

>>> from properties import Person
>>> trey = Person("Trey", "Hunner")
>>> trey.name
'Trey Hunner'
>>> trey.last_name = "Smith"
>>> trey.name
'Trey Smith'

Circle

This is the Circle exercise in properties.py.

Create the Circle class so that it:

can be constructed with an explicit radius or the default radius of 1
has an area property which auto-updates based on radius changes
has a diameter property which auto-updates based on radius changes
auto-updates the radius property based on diameter changes

Example usage:

>>> from properties import Circle
>>> circle1 = Circle()
>>> circle.radius
1
>>> circle.area
3.141592653589793
>>> circle.diameter
>>> circle.radius = 2
>>> circle.area
12.566370614359172
>>> circle.diameter
4
>>> circle.diameter = 10
>>> circle.radius
5.0
>>> circle.area
78.53981633974483

Log Radius Changes

This is the Circle exercise in properties.py.

Modify the Circle class we defined previously to use a property for the radius and record changes to the radius in a radius_changes attribute.

Note

To test these changes, you should modify the CircleTests class in properties_test.py to comment out the @unittest.skip for the lines from the appropriate test methods.

Hint

You will need to store the actual radius somewhere. You may want to use a _radius attribute when converting the radius to a property.

Example usage:

>>> from properties import Circle
>>> circle = Circle()
>>> circle.radius
1
>>> circle.radius_changes
[1]
>>> circle.radius = 2
>>> circle.radius = 3
>>> circle.radius_changes
[1, 2, 3]

Set Radius Error

This is the Circle exercise in properties.py.

Edit the Circle class and modify it again to raise a ValueError when the radius or diameter are set to a negative number.

Note

To test these changes, you should modify the CircleTests class in properties_test.py to comment out the @unittest.skip for the lines from the appropriate test methods.

Example usage:

>>> from properties import Circle
>>> circle = Circle()
>>> circle.radius = -10
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "shapes.py", line 18, in radius
    raise ValueError("Radius cannot be negative")
ValueError: Radius cannot be negative