Detailed explanation of python class instance analysis

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detailed explanation of python class

Class

1. A class is a data structure that can be used to create instances. (Generally, a class encapsulates data and methods that can be used for that data)

2. Python classes are callable objects , that is, class objects

3. Classes are usually defined at the top level of a module so that class instances can be created anywhere in the source code file where the class is defined.

4. Instance initialization

1. instance = ClassName(args....)

2. Classes can use two special methods, init and del, when instantiating them.

 class ClassName(base):
'class documentation string' #类文档字符串
 class suite        #类体

1. base: A superclass is a collection of one or more parent classes used for inheritance

2. The class body can include: declaration statements, class member definitions, data Attributes, methods

3. If the class does not have an inheritance relationship, the parentheses will not be provided. base

 class FirstClass():
  spam = 30    #类数据属性
  def display(self): #类方法
   print self.spam
 x = FirstClass()   #创建类实例
 x.display()     #方法调用
 >>> 30
 dir(FirstClass)
 >>> ['doc', 'module', 'display', 'spam']

• The class statement is similar to def and is executable code; the class will not be created until the class statement is run

• Within the class statement, any assignment statement will create class attributes

• Each instance object will inherit the attributes of the class and obtain its own namespace

Python class methods and calls

Attributes contained in the instance (object)

• Callable attributes: methods

• Data Attribute

In OOP, instances are like records with "data", and classes handle these records" Program”

• Calling a method through an instance is equivalent to calling a method of the class to which it belongs to process the current instance. For example, in the previous code example, x.display() will be automatically converted to FirstClass.display(x), that is, the method of the class is called to process the instance x

• . Therefore, each method in the class There must be a self parameter, which implies the meaning of the current instance

• Assigning the self attribute within the method will generate its own attributes for each instance

• Python stipulates that methods are not allowed to be called without instances. This is the concept of 'binding'

• The assignment statement in the class statement will create class attributes, such as The spam of the previous example

• Assigning a value to the special parameter self passed to the method in the class method will create an instance attribute

Python Constructor

When creating an instance, Python will automatically call the init method in the class to provide attributes for the instance invisibly

1. The init method is called a constructor

2. If there is no init method defined in the class, the instance is initially created as a simple namespace.

3. The first parameter of init must be self, self Variable is used to reference the instance to which the method is bound in the class instance method. Because an instance of a method is always passed as the first argument in any method call, self was chosen to represent the instance. You must put self in the method declaration, but you can use the method without an instance (self). If you don't use self in your method, then consider creating a regular function unless you have a specific reason. After all, your method code is not using an instance and has no functionality associated with the class, which makes it look more like a regular function. In other object-oriented languages, self might be called this.

• init cannot return any object

• ##Destructor: del

• The constructor is necessary, but the destructor can often be ignored (the Python interpreter will recycle it by itself)

class MyClass():
 def init(self, name):
  self.name = name
  print 'My name is ' + self.name
 def del(self):
  print self.name + ' is dead.'
i1 = MyClass('Shaw')
>>> My name is Shaw
del i1
>>> Shaw id dead.

类的特殊属性

• 使用dir()或dict，查看类或实例的属性

• doc：获取文档字符串

• base：获取所有父类

• module：类所在的模块

• name：实例所属类的名字

Python类方法中可用的变量

• 实例变量：self.变量名

• 局部变量：方法内部创建的变量，可直接使用

• 静态变量：类中定义的变量。类名.变量名

• 全局变量：直接使用

继承

继承描述了基类的属性如何‘遗传'给派生类

1. 子类可以继承他的基类的任何属性，包括数据属性和方法

2. 一个未指定基类的类，其默认有一个叫object的基类

3. Python允许多重继承（可以继承多个父类）

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