2.7. Classes

2.7.1. Abstract Classes: Declare Methods without Implementation

Sometimes you might want different classes to use the same attributes and methods. But the implementation of those methods can be slightly different in each class.

A good way to implement this is to use abstract classes. An abstract class contains one or more abstract methods.

An abstract method is a method that is declared but contains no implementation. The abstract method requires subclasses to provide implementations.

from abc import ABC, abstractmethod 

class Animal(ABC):

    def __init__(self, name: str):
        self.name = name 
        super().__init__()

    @abstractmethod 
    def make_sound(self):
        pass 

class Dog(Animal):
    def make_sound(self):
        print(f'{self.name} says: Woof')

class Cat(Animal):
    def make_sound(self):
        print(f'{self.name} says: Meows')

Dog('Pepper').make_sound()
Cat('Bella').make_sound()
Pepper says: Woof
Bella says: Meows

2.7.2. classmethod: What is it and When to Use it

When working with a Python class, if you want to create a method that returns that class with new attributes, use classmethod.

Classmethod doesn’t depend on the creation of a class instance. In the code below, I use classmethod to instantiate a new object whose attribute is a list of even numbers.

class Solver:
    def __init__(self, nums: list):
        self.nums = nums
    
    @classmethod
    def get_even(cls, nums: list):
        return cls([num for num in nums if num % 2 == 0])
    
    def print_output(self):
        print("Result:", self.nums)

# Not using class method       
nums = [1, 2, 3, 4, 5, 6, 7]
solver = Solver(nums).print_output()
Result: [1, 2, 3, 4, 5, 6, 7]
solver2 = Solver.get_even(nums)
solver2.print_output()
Result: [2, 4, 6]

2.7.3. getattr: a Better Way to Get the Attribute of a Class

If you want to get a default value when calling an attribute that is not in a class, use getattr() method.

The getattr(class, attribute_name) method simply gets the value of an attribute of a class. However, if the attribute is not found in a class, it returns the default value provided to the function.

class Food:
    def __init__(self, name: str, color: str):
        self.name = name
        self.color = color


apple = Food("apple", "red")

print("The color of apple is", getattr(apple, "color", "yellow"))
The color of apple is red
print("The flavor of apple is", getattr(apple, "flavor", "sweet"))
The flavor of apple is sweet
print("The flavor of apple is", apple.sweet)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
/tmp/ipykernel_337430/3178150741.py in <module>
----> 1 print("The flavor of apple is", apple.sweet)

AttributeError: 'Food' object has no attribute 'sweet'

2.7.4. __call__: Call your Class Instance like a Function

If you want to call your class instance like a function, add __call__ method to your class.

class DataLoader:
    def __init__(self, data_dir: str):
        self.data_dir = data_dir
        print("Instance is created")

    def __call__(self):
        print("Instance is called")


data_loader = DataLoader("my_data_dir")
# Instance is created

data_loader()
# Instance is called
Instance is created
Instance is called

2.7.5. Static method: use the function without adding the attributes required for a new instance

Have you ever had a function in your class that doesn’t access any properties of a class but fits well in a class? You might find it redundant to instantiate the class to use that function. That is when you can turn your function into a static method.

All you need to turn your function into a static method is the decorator @staticmethod. Now you can use the function without adding the attributes required for a new instance.

import re


class ProcessText:
    def __init__(self, text_column: str):
        self.text_column = text_column

    @staticmethod
    def remove_URL(sample: str) -> str:
        """Replace url with empty space"""
        return re.sub(r"http\S+", "", sample)


text = ProcessText.remove_URL("My favorite page is https://www.google.com")
print(text)
My favorite page is 

2.7.6. Property Decorator: A Pythonic Way to Use Getters and Setters

If you want users to use the right data type for a class attribute or prevent them from changing that attribute, use the property decorator.

In the code below, the first color method is used to get the attribute color and the second color method is used to set the value for the attribute color.

class Fruit:
    def __init__(self, name: str, color: str):
        self._name = name
        self._color = color

    @property
    def color(self):
        print("The color of the fruit is:")
        return self._color

    @color.setter
    def color(self, value):
        print("Setting value of color...")
        if self._color is None:
            if not isinstance(value, str):
                raise ValueError("color must be of type string")
            self.color = value
        else:
            raise AttributeError("Sorry, you cannot change a fruit's color!")


fruit = Fruit("apple", "red")
fruit.color
The color of the fruit is:
'red'
fruit.color = "yellow"
Setting value of color...
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
/tmp/ipykernel_337430/2513783301.py in <module>
----> 1 fruit.color = "yellow"

/tmp/ipykernel_337430/2891187161.py in color(self, value)
     17             self.color = value
     18         else:
---> 19             raise AttributeError("Sorry, you cannot change a fruit's color!")
     20 
     21 

AttributeError: Sorry, you cannot change a fruit's color!

2.7.7. __str__ and __repr__: Create a String Representation of a Python Object

If you want to create a string representation of an object, add __str__ and __repr__.

__str__ shows readable outputs when printing the object. __repr__ shows outputs that are useful for displaying and debugging the object.

class Food:
    def __init__(self, name: str, color: str):
        self.name = name
        self.color = color

    def __str__(self):
        return f"{self.color} {self.name}"

    def __repr__(self):
        return f"Food({self.color}, {self.name})"


food = Food("apple", "red")

print(food)  #  str__
red apple
food  # __repr__
Food(red, apple)