Python OOP

I. What is Python OOP?

OOP = Object Oriented Programming.

• Python fully supports OOP with Class and Class Inheritance. A Python class is a blueprint for creating from 1 to many objects. It can have:
• Attributes are attached to it to maintain its state. They may be read-only or writeable.
• Methods to modify its state. Syntax to define a class name “Animal”:

<span>class</span> <span>Animal</span><span>:</span>
    <span>def</span> <span>__init__</span><span>(</span><span>self</span><span>,</span> <span>secret_attribute</span><span>=</span><span>"</span><span>Suyt</span><span>"</span><span>):</span>
        <span># __init__: constructor that runs when an object is created. </span>        <span># self: refers to the current instance of the class </span>        <span>self</span><span>.</span><span>num_eyes</span> <span>=</span> <span>2</span>
        <span>self</span><span>.</span><span>__secret_attribute</span> <span>=</span> <span>secret_attribute</span> <span># Private variable </span>
    <span>def</span> <span>breathe</span><span>(</span><span>self</span><span>):</span>
        <span>print</span><span>(</span><span>"</span><span>Inhale, exhale.</span><span>"</span><span>)</span>
    <span>def</span> <span>get_secret_attribute</span><span>(</span><span>self</span><span>):</span>
        <span>return</span> <span>self</span><span>.</span><span>__secret_attribute</span>
    <span>def</span> <span>make_sound</span><span>(</span><span>self</span><span>):</span>
        <span>pass</span>  <span># To be implemented by subclasses </span>
<span>class</span> <span>Animal</span><span>:</span>
    <span>def</span> <span>__init__</span><span>(</span><span>self</span><span>,</span> <span>secret_attribute</span><span>=</span><span>"</span><span>Suyt</span><span>"</span><span>):</span>
        <span># __init__: constructor that runs when an object is created. </span>        <span># self: refers to the current instance of the class </span>        <span>self</span><span>.</span><span>num_eyes</span> <span>=</span> <span>2</span>
        <span>self</span><span>.</span><span>__secret_attribute</span> <span>=</span> <span>secret_attribute</span> <span># Private variable </span>
    <span>def</span> <span>breathe</span><span>(</span><span>self</span><span>):</span>
        <span>print</span><span>(</span><span>"</span><span>Inhale, exhale.</span><span>"</span><span>)</span>

    <span>def</span> <span>get_secret_attribute</span><span>(</span><span>self</span><span>):</span>
        <span>return</span> <span>self</span><span>.</span><span>__secret_attribute</span>

    <span>def</span> <span>make_sound</span><span>(</span><span>self</span><span>):</span>
        <span>pass</span>  <span># To be implemented by subclasses </span>
class Animal:
    def __init__(self, secret_attribute="Suyt"):
        # __init__: constructor that runs when an object is created.         # self: refers to the current instance of the class         self.num_eyes = 2
        self.__secret_attribute = secret_attribute # Private variable 
    def breathe(self):
        print("Inhale, exhale.")

    def get_secret_attribute(self):
        return self.__secret_attribute

    def make_sound(self):
        pass  # To be implemented by subclasses 

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II. What are the core concepts or elements of OOPs?

• Class: a blueprint for creating objects class Animal
• Object: an instance of a class. Instantiation is an action that creates an instance from the class nemo_fish = Fish().

• Encapsulation: restricted direct access to attributes or from the outside self.__secret_attribute

• Inheritance: Allows a child class to use methods and properties from parents’ classes.

<span>class</span> <span>Fish</span><span>(</span><span>Animal</span><span>):</span>
    <span>"""</span><span>Fish class is an inheritance from Animal</span><span>"""</span>
    <span>def</span> <span>__init__</span><span>(</span><span>self</span><span>):</span>
        <span>super</span><span>().</span><span>__init__</span><span>()</span> <span># call Animal init </span>
    <span>def</span> <span>breathe</span><span>(</span><span>self</span><span>):</span>
        <span>super</span><span>().</span><span>breathe</span><span>()</span> <span>#call the Animal breathe function, then modify it. </span>        <span>print</span><span>(</span><span>"</span><span>doing this underwater.</span><span>"</span><span>)</span>
    <span>def</span> <span>swim</span><span>(</span><span>self</span><span>):</span>
        <span>print</span><span>(</span><span>"</span><span>moving in the water</span><span>"</span><span>)</span>
<span>class</span> <span>Fish</span><span>(</span><span>Animal</span><span>):</span>
    <span>"""</span><span>Fish class is an inheritance from Animal</span><span>"""</span>
    <span>def</span> <span>__init__</span><span>(</span><span>self</span><span>):</span>
        <span>super</span><span>().</span><span>__init__</span><span>()</span> <span># call Animal init </span>
    <span>def</span> <span>breathe</span><span>(</span><span>self</span><span>):</span>
        <span>super</span><span>().</span><span>breathe</span><span>()</span> <span>#call the Animal breathe function, then modify it. </span>        <span>print</span><span>(</span><span>"</span><span>doing this underwater.</span><span>"</span><span>)</span>

    <span>def</span> <span>swim</span><span>(</span><span>self</span><span>):</span>
        <span>print</span><span>(</span><span>"</span><span>moving in the water</span><span>"</span><span>)</span>
class Fish(Animal):
    """Fish class is an inheritance from Animal"""
    def __init__(self):
        super().__init__() # call Animal init 
    def breathe(self):
        super().breathe() #call the Animal breathe function, then modify it.         print("doing this underwater.")

    def swim(self):
        print("moving in the water")

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• Polymorphism: allows different classes to use the same method name but different implementations.

<span>class</span> <span>Dog</span><span>(</span><span>Animal</span><span>):</span>
    <span>def</span> <span>make_sound</span><span>(</span><span>self</span><span>):</span>
        <span>return</span> <span>"</span><span>Woof!</span><span>"</span>
<span>class</span> <span>Cat</span><span>(</span><span>Animal</span><span>):</span>
    <span>def</span> <span>make_sound</span><span>(</span><span>self</span><span>):</span>
        <span>return</span> <span>"</span><span>Meow!</span><span>"</span>
<span>animals</span> <span>=</span> <span>[</span><span>Dog</span><span>(),</span> <span>Cat</span><span>()]</span>
<span>for</span> <span>animal</span> <span>in</span> <span>animals</span><span>:</span>
    <span>print</span><span>(</span><span>animal</span><span>.</span><span>make_sound</span><span>())</span>  <span># Calls the correct method based on the object type </span>
<span>class</span> <span>Dog</span><span>(</span><span>Animal</span><span>):</span>
    <span>def</span> <span>make_sound</span><span>(</span><span>self</span><span>):</span>
        <span>return</span> <span>"</span><span>Woof!</span><span>"</span>

<span>class</span> <span>Cat</span><span>(</span><span>Animal</span><span>):</span>
    <span>def</span> <span>make_sound</span><span>(</span><span>self</span><span>):</span>
        <span>return</span> <span>"</span><span>Meow!</span><span>"</span>

<span>animals</span> <span>=</span> <span>[</span><span>Dog</span><span>(),</span> <span>Cat</span><span>()]</span>
<span>for</span> <span>animal</span> <span>in</span> <span>animals</span><span>:</span>
    <span>print</span><span>(</span><span>animal</span><span>.</span><span>make_sound</span><span>())</span>  <span># Calls the correct method based on the object type </span>
class Dog(Animal):
    def make_sound(self):
        return "Woof!"

class Cat(Animal):
    def make_sound(self):
        return "Meow!"

animals = [Dog(), Cat()]
for animal in animals:
    print(animal.make_sound())  # Calls the correct method based on the object type 

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• Abstraction: hiding implementation details and only exposing essential functionalities. For example: we have many 2 kinds of animals: cats, dogs, and fish. We know they care how they make the sound, only care that they can make a sound.

<span>from</span> <span>abc</span> <span>import</span> <span>ABC</span><span>,</span> <span>abstractmethod</span> <span># import abstract base class module </span>
<span>class</span> <span>Animal</span><span>(</span><span>ABC</span><span>):</span>
    <span>@abstractmethod</span>
    <span>def</span> <span>make_sound</span><span>(</span><span>self</span><span>):</span>
        <span>pass</span>
<span># Getting error if instantiate an abstract class # dog = Animal() # TypeError: Can't instantiate abstract class Animal # without an implementation for abstract method 'make_sound' </span>
<span># Dog class inherits from Animal so it has to implement method make_sound </span><span>class</span> <span>Dog</span><span>(</span><span>Animal</span><span>):</span>
    <span>def</span> <span>make_sound</span><span>(</span><span>self</span><span>):</span>
        <span>return</span> <span>"</span><span>Woof! Woof!</span><span>"</span>
<span>perfect_dog</span> <span>=</span> <span>Dog</span><span>()</span>
<span>print</span><span>(</span><span>perfect_dog</span><span>.</span><span>make_sound</span><span>())</span>
<span># Woof! Woof! </span>
<span>from</span> <span>abc</span> <span>import</span> <span>ABC</span><span>,</span> <span>abstractmethod</span> <span># import abstract base class module </span>
<span>class</span> <span>Animal</span><span>(</span><span>ABC</span><span>):</span>
    <span>@abstractmethod</span>
    <span>def</span> <span>make_sound</span><span>(</span><span>self</span><span>):</span>
        <span>pass</span>

<span># Getting error if instantiate an abstract class # dog = Animal() # TypeError: Can't instantiate abstract class Animal # without an implementation for abstract method 'make_sound' </span>
<span># Dog class inherits from Animal so it has to implement method make_sound </span><span>class</span> <span>Dog</span><span>(</span><span>Animal</span><span>):</span>
    <span>def</span> <span>make_sound</span><span>(</span><span>self</span><span>):</span>
        <span>return</span> <span>"</span><span>Woof! Woof!</span><span>"</span>

<span>perfect_dog</span> <span>=</span> <span>Dog</span><span>()</span>
<span>print</span><span>(</span><span>perfect_dog</span><span>.</span><span>make_sound</span><span>())</span>
<span># Woof! Woof! </span>
from abc import ABC, abstractmethod # import abstract base class module 
class Animal(ABC):
    @abstractmethod
    def make_sound(self):
        pass

# Getting error if instantiate an abstract class # dog = Animal() # TypeError: Can't instantiate abstract class Animal # without an implementation for abstract method 'make_sound' 
# Dog class inherits from Animal so it has to implement method make_sound class Dog(Animal):
    def make_sound(self):
        return "Woof! Woof!"

perfect_dog = Dog()
print(perfect_dog.make_sound())
# Woof! Woof! 

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原文链接：Python OOP