Of Stack

Basic Explanation and Application of python Class

  • 2021-12-12 08:53:45
  • OfStack
Directory class definition class object variable class variable local variable instance variable private variable class method special method inheritance single inheritance multiple inheritance

Definition of class 


# class Is the keyword that defines the class, ClassName Is the name of the class 
class ClassName:
	#  Write something else here 
	pass

class ClassName(object):
	#  Write something else here 
	pass

This is the simplest class definition. After python3, class definition 1 generally inherits object class, but it doesn't have much problem if it doesn't inherit.

Class object

Class objects are also called instances 


#  This is the creation of the 1 Class objects (instances), a Yes 1 Instances are also 1 Objects 
a = ClassName()

Variable

There are: class variables, local variables, instance variables inside the class

Class variable

Class variables: All instantiated objects of a class share class variables at the same time, that is, class variables exist as common resources among all instantiated objects. 


class ClassName(object):
	#  Definition 1 Individual class variable 
	name = "Tom"

There are two ways to call a class method, either directly using the class name or using the instantiation (object) of the class. The value of a class variable can be modified by the class name, and after modification, all instantiated objects will be affected. 


#  Called directly by class name 
print(ClassName.name)
#  Create an instance of a class (object) 
a = ClassName()
b = ClassName()
//  Called through an instance of the class 
print(a.name)
print(b.name)
ClassName.name = "Tim"
print(a.name)
print(b.name)

The running result is:

Tom
Tom
Tom
Tim
Tim

However, if the value of the class variable of an instance is modified, the value of the variable is modified by the class name, which will not affect the class variable of this instance. 


a = ClassName()
b = ClassName()
a.name = "asf"
ClassName.name = "Pig"
print(a.name)
print(b.name)
ClassName.name = "aaa"
print(a.name)
print(b.name)

Run results:

asf
Pig
asf
aaa

Through the running results, it can be seen that after the object a changes the value of name, it will call Class Name.name = "Pig" The name variable value of the object a does not change

Local variable

Local variables: Local variables are only defined within class methods and can only be used within methods 


class ClassName(object):
	#  This is 1 Methods 
	def function(self):
		#  This is 1 Local variables 
		name = "Tom"

name is defined within the class method, so it is a local variable, which cannot be called externally and can only be used within the method.

Instance variable

Instance variable: A variable defined as an self. variable name, called an instance variable (attribute) 


class ClassName(object):
	#  This is 1 Instance variables 
	def function1(self):
		self.name = "Tom"
	def function2(self):
		print(self.name)

It only works on the object that calls the method, and can be called within other methods. 


className = ClassName()
className.function()

Run results:

Tom

If defined in the __init__() The instance variable in the method can be accessed only by the object name. 


class ClassName(object):
	def __init__():
		self.name = "Tom"

className = ClassName()
print(className.name)

Run results:

Tom

Private variable

__name This is the private variable. Add two before the variable _ , self.__name It is also a private variable, which cannot be accessed directly outside the class. 


class ClassName(object):
	#  Defines the 1 Private class variables 
	__name = "Tom"
    
ClassName.__name

Run results:

---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
< ipython-input-8-08830fea0534 > in < module >
2 __name = "Tom"
3
---- > 4 ClassName.__name

AttributeError: type object 'ClassName' has no attribute '__name'

Methods of the class

Use in the class def Keyword to define methods, also called functions. The difference with the ordinary method is that you must need 1 self Parameters of, self Represents the class itself 


#  This is the creation of the 1 Class objects (instances), a Yes 1 Instances are also 1 Objects 
a = ClassName()
0

def To define the keywords of the function, function Is the name of the function,

Special method

1. __init__() Parameter 1 used to initialize a class is generally called a constructor
This method is used by all classes. 


#  This is the creation of the 1 Class objects (instances), a Yes 1 Instances are also 1 Objects 
a = ClassName()
1

When the class is instantiated, it is automatically called __init__

2. __call__() Called when the object is treated as a function. 


#  This is the creation of the 1 Class objects (instances), a Yes 1 Instances are also 1 Objects 
a = ClassName()
2

3. __get__() Class is called automatically when it is a property of another class 


class ClassName:
	def __init__(self):
		pass
	def __get__(self,instance, owner):
		print("__get__ Called ")

class ClassName2:
		a = ClassName()

Run results:

__get__ calls the

4. __new__() Is a static method responsible for creating class instances, and this method takes precedence __init__() The initialization method is called.

. . . There are many other class-specific methods

Inheritance

Inheritance is to obtain the property of parents in life, and to obtain all variables and methods of parents in programming languages. Deriving a new class from a class by inheritance

Single inheritance

In short, it inherits 1 class, and the inherited class is the base class (parent class) and the successor is a subclass 


#  This is the creation of the 1 Class objects (instances), a Yes 1 Instances are also 1 Objects 
a = ClassName()
4

The above is a simple inheritance, and the subclass writes the name of the class to inherit in parentheses after the class name. 


#  This is the creation of the 1 Class objects (instances), a Yes 1 Instances are also 1 Objects 
a = ClassName()
5

Run results:

Tom
..

It is found that the B class does not write any variables and methods, but calls the variables (properties) and methods of the A class
Of course, if you don't want a variable or method of the parent class, you can override this variable or method 


#  This is the creation of the 1 Class objects (instances), a Yes 1 Instances are also 1 Objects 
a = ClassName()
6

Run results:

Tom
bbb

Multiple inheritance

In terms of name, multiple inheritance inherits multiple classes 


#  This is the creation of the 1 Class objects (instances), a Yes 1 Instances are also 1 Objects 
a = ClassName()
7

The above C class inherits both A and B classes

Related articles: