Python's sample tutorial on the use of class variables and member variables

This article illustrates the usage of python class variables and member variables in the form of an example, which has certain reference value for python programming. Share with you for your reference. The details are as follows:

Take a look at this code:

class TestClass(object):
  val1 = 100
  
  def __init__(self):
    self.val2 = 200
  
  def fcn(self,val = 400):
    val3 = 300
    self.val4 = val
    self.val5 = 500 
 if __name__ == '__main__':
  inst = TestClass()
   
  print TestClass.val1
  print inst.val1
  print inst.val2
  print inst.val3
  print inst.val4  
  print inst.val5

Here, val1 is a class variable that can be called either directly by the class name or by an object.
Val2 is a member variable, which can be called by the object of the class. Here we can see that the member variable must be given in the form of self.
Val3 is not a member variable, but a local variable inside the function FCN.
Neither val4 nor val5 are member variables, given as self.but not initialized in the constructor.

Take a look at the following code (after the # sign is the result of the run) :

inst1 = TestClass()
inst2 = TestClass()

print TestClass.val1 # 100
print inst1.val1   # 100

inst1.val1 = 1000  
print inst1.val1   # 1000
print TestClass.val1 # 100

TestClass.val1 =2000 
print inst1.val1   # 1000
print TestClass.val1 # 2000

print inst2.val1   # 2000   

inst3 = TestClass() 
print inst3.val1   # 2000

You can see that python class variables are different from C++ static variables and are not Shared by all objects in the class. The class itself has its own class variable (stored in memory). When the object of a TestClass class is constructed, it will copy the current class variable to the object. What is the value of the current class variable? Moreover, modifying a class variable through an object does not affect the value of a class variable of another object, because everyone has their own copy, and it does not affect the value of the class variable owned by the class itself. Only the class itself can change the value of a class variable owned by the class itself.

Hopefully, the examples described in this article will help you understand and master the usage of Python's class and member variables.