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)
2.7.8. attrs: Bring Back the Joy of Writing Classes¶
!python -m pip install attrs
Do you find it annoying to write an __init__ method every time you want to create a class in Python?
class Dog:
def __init__(self, age: int, name: str, type_: str = 'Labrador Retriever'):
self.age = age
self.name = name
self.type_ = type_
def describe(self):
print(f"{self.name} is a {self.type_}.")
If so, try attrs. With attrs, you can declaratively define the attributes of a class.
import attr
@attr.s(auto_attribs=True)
class Dog:
age: int
name: str
type_: str = "Labrador Retriever"
def describe(self):
print(f"{self.name} is a {self.type_}.")
pepper = Dog(7, "Pepper", "Labrador Retriever")
The instance created using attrs has a nice human-readable __repr__.
pepper
Dog(age=7, name='Pepper', type_='Labrador Retriever')
pepper.describe()
Pepper is a Labrador Retriever.
You can also turn the attributes of that instance into a dictionary.
attr.asdict(pepper)
{'age': 7, 'name': 'Pepper', 'type_': 'Labrador Retriever'}
You can also compare 2 instances of the same class using the first attribute of that class.
bim = Dog(8, 'Bim Bim', 'Dachshund')
pepper < bim
True
Find other benefits of attrs here.