Decorators, class methods, and properties

Overview

Teaching: 20 min
Exercises: 25 min

Questions

• What is a decorator?

• How do I tag methods as being applicable to a class rather than an instance?

• How can I add logic to process changes to instance variables?

Objectives

• Understand the purpose of decorators and how they are implemented

• Be able to use @classmethod and @property

Sometimes when we are writing software we would like to be able to attach additional functionality to a variety of functions (or classes) without writing the functionality directly into the function. Python gives us some extra syntax to make this easier.

For example, say that we want to track what functions are being called in our program. We can write a function that will take a function that we want to track as an argument, and return a new function that outputs before and after calling the function we are interested in.

def track_this(function):
    def new_function():
        print("Entering", function)
        function()
        print("Leaving", function)

    return new_function

To test this out:

def say_hello():
    print("Hello, world.")

say_hello = track_this(say_hello)

def say_goodbye():
    print("See you later.")

say_goodbye = track_this(say_goodbye)

def conversation():
    say_hello()
    say_goodbye()

conversation = track_this(conversation)

conversation()

Entering <function conversation at 0x1115c07b8>
Entering <function say_hello at 0x11143be18>
Hello, world.
Leaving <function say_hello at 0x11143be18>
Entering <function say_goodbye at 0x1115c0e18>
See you later.
Leaving <function say_goodbye at 0x1115c0e18>
Leaving <function conversation at 0x1115c07b8>

So we can now see in more detail what’s going on as we move through this program. However, having to set each function to the result of calling track_this on the function is laborious; it would be nicer if there were an easier way to do this, and thankfully Python gives us one.

@track_this
def say_hello():
    print("Hello, world.")

@track_this
def say_goodbye():
    print("See you later.")

@track_this
def conversation():
    say_hello()
    say_goodbye()

conversation()

Using @ followed by the name of the altering function (track_this in this case), and placing this before the function definition, Python takes the result of calling the altering function and overwrites the new function with it.

This syntax is called a “decorator”; the functions say_hello, say_goodbye, and conversation have been decorated with the track_this decorator.

Our @track_this decorator is currently not very general. We can see a problem when we try and decorate a function that takes arguments:

Decorators and arguments

@track_this
def say_something(thing_to_say):
    print(thing_to_say)

say_something("Hello there")

TypeError                                 Traceback (most recent call last)
<ipython-input-29-000c7283eed1> in <module>()
      3     print(thing_to_say)
      4
----> 5 say_something("Hello there")

TypeError: new_function() takes 0 positional arguments but 1 was given

To make this more flexible, we can rewrite the track_this decorator as:

def track_this(function):
    def new_function(*args, **kwargs):
        print("Entering", function)
        function(*args, **kwargs)
        print("Leaving", function)

    return new_function

The * and ** here carry two meanings. In the definition def new_function(*args, **kwargs), they mean “take any positional arguments and put them into a list called args, and take any keyword arguments and put them into a dict called kwargs. In the function call function(*args, **kwargs), they mean “pass each element of the list args as a separate argument, and pass each element of the dict kwargs as a keyword argument.

You can also write and use decorators that themselves accept arguments by using a nested function definition, but we won’t go into detail about this today.

Double checking

Try writing a decorator that checks the result of a computation is consistent by running it twice and checking the outputs are equal. This should return the result if it is consistent, and raise an exception otherwise. Test it by decorating the area and perimeter methods of the Polygon and Triangle classes from the previous episode.

Solution

class InconsistentResultsError(AssertionError):
    pass


def check_consistency(function):
    def consistent_function(*args, **kwargs):
        results = [function(*args, **kwargs) for _ in range(2)]
        if results[0] != results[1]:
            raise InconsistentResultsError
        return results[0]

    return consistent_function


class Polygon:
    def __init__(self, side_lengths):
        filtered_side_lengths = []
        for side_length in side_lengths:
            assert side_length >= 0
            if side_length > 0:
                filtered_side_lengths.append(side_length)
        self.side_lengths = filtered_side_lengths

    @check_consistency
    def perimeter(self):
        '''Returns the perimeter of the polygon.'''
        return sum(self.side_lengths)

class Triangle(Polygon):
    def __init__(self, side_lengths):
        # Triangles have three sides
        super().__init__(side_lengths)
        assert len(self.side_lengths) == 3

    @check_consistency
    def area(self):
        '''Returns the area of the triangle.'''
        a, b, c = self.side_lengths
        p = (a + b + c) / 2
        return (p * (p - a) * (p - b) * (p - c)) ** 0.5

a_triangle = Triangle([3, 4, 5])
print("Perimeter:", a_triangle.perimeter())
print("Area:", a_triangle.area())

Class methods

Sometimes we want to write functions associated with classes that are relevant to the class as a whole, rather than to one specific instance. We can do this by adding the @classmethod decorator to a method.

Class methods are most frequently used as specialised constructors, to create instances of the class without having to supply every argument to __init__.

For example, revisiting the Triangle class from earlier, we may want to be able to define an equilateral triangle by giving a single side length.

class Triangle(Polygon):
    def __init__(self, side_lengths):
        # Triangles have three sides
        super().__init__(side_lengths)
        assert len(self.side_lengths) == 3

    @classmethod
    def equilateral(cls, side_length):
        return cls([side_length] * 3)

    def area(self):
        '''Returns the area of the triangle.'''
        a, b, c = self.side_lengths
        p = (a + b + c) / 2
        return (p * (p - a) * (p - b) * (p - c)) ** 0.5

Notice that in addition to adding the @classmethod decorator, the first argument which is usually self has been replaced with cls. Since class methods aren’t specific to a particular instance, there is no need to have the self argument referring to it. Conversely, it is useful to be able to refer to the specific class without having to do this by name, since in general we would like class methods to work and return the correct type of class for subclasses as well.

Let’s test this now.

e_triangle = Triangle.equilateral(1.5)
print("Perimeter:", e_triangle.perimeter())
print("Area:", e_triangle.area())

Perimeter: 4.5
Area: 0.9742785792574935

Now we only need to supply a single number, the length of the side, and the equilateral class method constructs the list of three equal side lengths from this, returning a Triangle with three equal sides.

Squares

Add a class method to the Rectangle class which you wrote in the previous episode to create a square, given the length of its side.

Solution

class Rectangle(Polygon):
    def __init__(self, side_lengths):
        super().__init__(side_lengths)
        num_sides = len(self.side_lengths)
        assert num_sides == 2 or num_sides == 4
        if num_sides == 2:
            width, height = side_lengths
            self.side_lengths = [width, height, width, height]
        else:
            assert self.side_lengths[0] == self.side_lengths[2]
            assert self.side_lengths[1] == self.side_lengths[3]

    def area(self):
        return self.side_lengths[0] * self.side_lengths[1]

    @classmethod
    def square(cls, side_length):
        return cls([side_length] * 4)

Properties

In general, when working with classes, there is an assumption that instance variables can be modified, unless something is done to prevent this. In some languages, variables can be defined as read-only, or private so that they cannot be seen from outside of the class. Python has neither of these—any instance variable can be modified by any piece of code using the class. There is, however, a convention that variables and methods whose names begin with _ are private to the implementation—while they can be accessed from outside the class, they are not guaranteed to remain stable between versions, and the class doesn’t guarantee to behave well if they are changed.

To look at a specific example, what happens if we take the Triangle class and change side_lengths?

a_triangle = Triangle([3, 4, 5])
a_triangle.side_lengths = [3, 4, 5, 6]
print(a_triangle.area())

     11     def area(self):
     12         '''Returns the area of the triangle.'''
---> 13         a, b, c = self.side_lengths
     14         p = (a + b + c) / 2
     15         return (p * (p - a) * (p - b) * (p - c)) ** 0.5

ValueError: too many values to unpack (expected 3)

Our implementation of area assumes that side_lengths was validated by __init__ and so has three elements, all positive. By adding a fourth, the implementation becomes broken as a list of four elements can’t be unpacked to three variables. Similarly,

a_polygon = Polygon([1, 2, 3, 4, 5])
a_polygon.side_lengths = 'spam and eggs'
a_polygon.perimeter()

TypeError                                 Traceback (most recent call last)
<ipython-input-50-9b8ffd74de36> in <module>()
      1 a_polygon = Polygon([1, 2, 3, 4, 5])
      2 a_polygon.side_lengths = 'spam and eggs'
----> 3 a_polygon.perimeter()

<ipython-input-49-5ae60040e7be> in perimeter(self)
     10     def perimeter(self):
     11         '''Returns the perimeter of the polygon.'''
---> 12         return sum(self.side_lengths)
     13
     14

TypeError: unsupported operand type(s) for +: 'int' and 'str'

It doesn’t make sense to take the sum of a string (or more precisely, to add the individual characters together), so this also fails.

One way to fix this is to signal that this shouldn’t happen is to mark side_lengths as private by renaming it to _side_lengths. However, this removes some potentially useful functionality—it would definitely be useful for a user of the class to be able to read the side lengths, just not write them directly. Python provides us with an @property decorator that lets us do this.

class Polygon:
    def __init__(self, side_lengths):
        filtered_side_lengths = []
        for side_length in side_lengths:
            assert side_length >= 0
            if side_length > 0:
                filtered_side_lengths.append(side_length)
        self._side_lengths = filtered_side_lengths

    def perimeter(self):
        '''Returns the perimeter of the polygon.'''
        return sum(self._side_lengths)

    @property
    def side_lengths(self):
        return self._side_lengths

We have done two things here: self.side_lengths has been renamed to self._side_lengths, indicating that it is intended to be considered as private to the class. We have also added a new method side_lengths, and decorated that with the @property decorator. This allows the result of calling this function to be accessed as though it were an instance variable:

a_polygon = Polygon([1, 2, 3, 4, 5])
print(a_polygon.side_lengths)

[1, 2, 3, 4, 5]

However, we can’t assign to it without referring to the private _side_lengths:

a_polygon.side_lengths = 'spam and eggs'

AttributeError                            Traceback (most recent call last)
<ipython-input-57-9b8ffd74de36> in <module>()
      1 a_polygon = Polygon([1, 2, 3, 4, 5])
----> 2 a_polygon.side_lengths = 'spam and eggs'
      3 a_polygon.perimeter()

AttributeError: can't set attribute

So we have now successfully “protected” our class from changes that will break it, by signalling to users of it what is internal to the implementation, and what is designed for them to use. However, we have still removed a little functionality in the process—previously, a user could change side_lengths without breaking things, provided that they were careful (since the consistency checks of __init__ were being bypassed), whereas now this is not supported behaviour (even if it is possible).

What we would like is to offer the ability to set the value for the property, but add some kind of validation function that does that rather than allowing it to be assigned directly. This kind of function is called a setter, and the @property decorator in fact allows us to create one. (The first function, which gets the value, is referred to as a getter.)

class Polygon:
    def __init__(self, side_lengths):
        self.side_lengths = side_lengths

    def perimeter(self):
        '''Returns the perimeter of the polygon.'''
        return sum(self._side_lengths)

    @property
    def side_lengths(self):
        return self._side_lengths

    @side_lengths.setter
    def side_lengths(self, side_lengths):
        filtered_side_lengths = []
        for side_length in side_lengths:
            assert side_length >= 0
            if side_length > 0:
                filtered_side_lengths.append(side_length)
        self._side_lengths = filtered_side_lengths

We’ve moved the validation logic into the method side_length, as decorated by the @side_lengths.setter decorator, and the __init__ method uses this to do its initial setup. Testing this:

a_polygon = Polygon([1, 2, 3, 4, 5])
print("Original perimeter:", a_polygon.perimeter())
a_polygon.side_lengths = ([1, 2, 3, 4, 5, 6])
print("Modified perimeter:", a_polygon.perimeter())

Original perimeter: 15
Modified perimeter: 21

More robust plotters

Adjust the FunctionPlotter, PolynomialPlotter, or QuadraticPlotter example from earlier to make color a property, with a getter and a setter, with the setter checking that the the color is a valid matplotlib color.

Solution

from numpy import linspace
from matplotlib.pyplot import subplots
from matplotlib.colors import is_color_like

class FunctionPlotter:
    def __init__(self, color='red', linewidth=1, x_min=-10, x_max=10):
        self.color = color
        self.linewidth = linewidth
        self.x_min = x_min
        self.x_max = x_max

    @property
    def color(self):
        return self._color

    @color.setter
    def color(self, color):
        assert is_color_like(color)
        self._color = color

    def plot(self, function):
        '''Plot a function of a single argument.
        The line is plotted in the colour specified by color, and with width
        linewidth.'''
        fig, ax = subplots()
        x = linspace(self.x_min, self.x_max, 1000)
        ax.plot(x, function(x), color=self._color, linewidth=self.linewidth)

Key Points

• A decorator adds functionality to a class or function. To use the decoratorname decorator, add @decoratorname one line before the class or function definition.

• Use the @classmethod decorator to indicate methods to be called from the class rather than from an instance.

• Use the @property decorator to control access to instance variables