What keywords do Python use to define classes?

What is a class?

is used to describe a collection of objects with the same properties and methods. It defines the properties and methods common to every object in the collection. Objects are instances of classes.

What is the method?

The functions in the class are methods

How to define a class?

Define a class. The syntax format is as follows:

class ClassName:
    <statement-1>
    .
    .
    .
    <statement-N>

The class keyword is used in Python to define a class. The naming rule for classes is that the first letter of each word must be capitalized.

Class object

Class object supports two operations: attribute reference and instantiation.

Attribute references use the same standard syntax as all attribute references in Python: obj.name.

After the class object is created, all names in the class namespace are valid attribute names. So if the class definition is like this:

class MyClass:    """一个简单的类实例"""
    i = 12345
    def f(self):        return 'hello world'
 # 实例化类x = MyClass()
 # 访问类的属性和方法print("MyClass 类的属性 i 为：", x.i)print("MyClass 类的方法 f 输出为：", x.f())

The above creates a new class instance and assigns the object to the local variable x, x is an empty object.

The output result of executing the above program is:

MyClass 类的属性 i 为： 12345
MyClass 类的方法 f 输出为： hello world

The class has a special method (construction method) named __init__(), which is automatically called when the class is instantiated, as shown below This way:

def __init__(self):
    self.data = []

The class defines the __init__() method, and the instantiation operation of the class will automatically call the __init__() method. When the class MyClass is instantiated as follows, the corresponding __init__() method will be called:

x = MyClass()

Of course, the __init__() method can have parameters, and the parameters are passed to the instantiation operation of the class through __init__(). For example:

class Complex:
    def __init__(self, realpart, imagpart):
        self.r = realpart
        self.i = imagpart
x = Complex(3.0, -4.5)
print(x.r, x.i)   # 输出结果：3.0 -4.5

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