Python book learning notes - Chapter 09 section 01 create and use classes

9.1 Create and use classes

9.1.1 establish Dog class

   according to Dog Each instance created by the class will store the name and age , We gave each dog a squat (sit( )) And roll (roll_over( )) The ability of ：

class Dog:

    """ A simple attempt to simulate a puppy """

    def __init__(self, name, age):

        """ Initialization property name and age."""

        self.name = name

        self.age = age

    def sit(self):

        """ Simulate the dog squatting down after receiving the order """

        print(f'{self.name} is now sitting.')

    def roll_over(self):

        """ Simulate a dog rolling when ordered ."""

        print(f"{self.name} roll over!")

   Method __init__()

   A function in a class is called a method . Everything you learned about functions earlier applies to methods , For now , The only important difference is the way the method is called . The first 4 The way to go __init__() It's a special method , Every time you look at Dog Class when a new instance is created , Python Will run it automatically .

   In the name of the method , There are two underscores at the beginning and the end , It's a convention , To avoid Python The default method has a name conflict with the normal method . Make sure that __init__() On both sides of the Two Underline , Otherwise, when you use classes to create instances , This method will not be called automatically , Which leads to hard to find errors .

   We're going to approach __init__() Defined as containing three formal parameters ：self、name and age. In the definition of this method , Shape parameter self essential , And must In front of other formal parameters .

   Why must form parameters be included in the method definition self Well ？ because Python Call this method to create Dog When an instance , Arguments are automatically passed in self. Each method call associated with an instance automatically passes arguments self, It is a reference to the instance itself , Gives instances access to properties and methods in the class .

   establish Dog When an instance ,Python Will call Dog Class method __init__(). We're going to go through the argument direction Dog() Pass the name and age ,self It will automatically pass , So there's no need to pass it on . Whenever according to Dog Class when an instance is created , I'm just going to give you the last two parameters (name and age) Provide value .

   attribute

   The first 4 Both variables defined at line have prefixes self. With self Variables prefixed with are available to all methods in the class , Can be accessed through any instance of the class .self.name=name Get and formal parameters name Associated value , And assign it to a variable name, This variable is then associated with the currently created instance .self.age=age The effect is similar .

   Variables that can be accessed through instances like this are called attribute .

  Dog The class also defines two other methods ：sit() and roll_over(). These methods do not require additional information to execute , So they only have one parameter self.

9.1.2 Create an instance based on the class

   Create an instance that represents a particular dog ：

my_dog = Dog('Willie', 6)

print(f"My dog's name is {my_dog.name}.")

print(f"My dog is {my_dog.age} years old.")

output:

My dog's name is Willie.

My dog is 6 years old.

   The first 1 That's ok , Give Way Python Create a name of 'Willie', Age is 6 The little dog .

   When I run into this line of code ,Python Using the arguments 'Willie' and 6 call Dog Class method __init__(). Method __init__() Create an instance that represents a particular dog , And use the provided value to set the property name and age.

   Next ,Python Returns an instance representing the dog , And we assign this instance to the variable my_dog. ad locum , Naming conventions are useful ： Names that can usually be considered capitalized ( Such as Dog) Refers to the class , Lower case names ( Such as my_dog) Refers to an instance created from a class .

  a. Access properties —— Period notation

   The properties of the instance to access , Period notation can be used .

   Period notation Python In the common , This syntax is demonstrated Python How do I know the value of an attribute . ad locum ,Python Find the instance first my_dog, Then find the attribute associated with the instance name.

   stay Dog When this property is referenced in a class , It uses self.name and self.age.

  b. Calling method

   according to Dog Class after the instance is created , You can use the period notation to call Dog Any method defined in the class .

   Let's make the dog squat and roll ：

my_dog.sit()

my_dog.roll_over()

output:

Willie is now sitting.

Willie roll over!

   To invoke a method , The name of the instance can be specified （ Here is my_dog） And the method to call , And separated by a period .

   Meet the code my_dog.sit() when ,Python In the class Dog Find method in sit() And run the code .Python Interpret the code in the same way my_dog.roll_over().

   As we can see, the output ——Willie Did as we ordered .

  c. Create multiple instances

my_dog = Dog('Willie', 6)

your_dog = Dog('Lucy', 3)

print(f"My dog's name is {my_dog.name}.")

print(f"My dog is {my_dog.age} years old.")

my_dog.sit()

print(f"\nYour dog's name is {your_dog.name}.")

print(f"Your dog is {your_dog.age} years old.")

your_dog.sit()

output:

My dog's name is Willie.

My dog is 6 years old.

Willie is now sitting.

Your dog's name is Lucy.

Your dog is 3 years old.

Lucy is now sitting.

   In this example, two puppies are created , Respectively called Willie and Lucy. Every dog is a separate instance , It has its own set of attributes , Be able to perform the same operation .

   Even if the second puppy is given the same name and age ,Python Will still be based on Dog Class creates another instance . You can create as many instances of a class as you want , The condition is that each instance is stored in a different variable , Or occupy different places in a list or dictionary .

Exercises

Create a file called Restaurant Class , For its method __init__() Set properties restaurant_name and cuisine_type.

Create a file called describe_restaurant() And a method called open_restaurant() Methods , The former prints the above two information , The latter prints a message , Point out that the restaurant is open .

Create a class named restaurant Example , Print its two properties separately , Then call the above two methods .

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