Python static methods and class member methods

Python's static methods and class member methods can both be accessed by classes or instances. The concepts of the two are not easy to clarify, but there are still differences:

1) Static methods do not need to pass in the self parameter, and class member methods need to pass in the representative The cls parameter of this class; (the cls parameter represents this class)

2) From item 1, static methods cannot access instance variables, and class member methods also cannot access instance variables, but they can access class variables;

3) Static methods are a bit like function tool libraries, while class member methods are closer to static methods in Java object-oriented concepts.

Two ways to implement static methods and class methods

1. In Python 2.3 and before, use staticmethod and classmethod type object packaging to implement

The example is as follows (pay attention to the instructions in the print):

class MyClass:
val1 = 'Value 1'
         def __init__(self):
                                                                                                                                                                       def classmd(cls):
                                                                                                                                                                                           classmd (cls), classmd (cls), can’t access the value of val2.

>>> mc = MyClass()
>>> mc.smd()
>>> mc.cmd()
>>> MyClass.smd()
>>> MyClass.cmd()

2. In Python 2.4 and later, use decorators to implement

The decorator uses the @ operator. The example is as follows:

class MyClass:
val1 = 'Value 1'

  def __init__(self):

  self.val2 = 'Value 2'

  @staticmethod

  def staticmd():

  print 'Static method, cannot access val1 and val2'

 @classmethod
def classmd( cls):
    print 'Class method, class: ' + str(cls) + ', val1: ' + cls.val1 + ', cannot access the value of val2'


No matter which of the above two methods , the execution situation is the same. Taking the execution result of method 2 as an example, the analysis is as follows:

Execution:

>>> mc = MyClass() # Instantiate


>>> mc.staticmd () # Instance call static method, cannot access instance variables val1 and val2

>>>

Static method, cannot access val1 and val2

>>> mc.classmd() # Instance call Class method, please note that what is accessed here is the value of the variable val1 of the class MyClass, not the instance variable val1 of mc after instantiation. It is easy to be confused here. You will understand it if you read below. val2 has always been an instance variable, so it cannot be accessed

>>>

Class method, class: __main__.MyClass, val1: Value 1, the value of val2 cannot be accessed

>>> MyClass.staticmd () # The class directly calls the static method, the result is the same as the above instance call, neither the class variable nor the instance variable can be accessed

>>>

Static method, val1 and val2 cannot be accessed

>> ;> MyClass.classmd() # The class directly calls the class method, the result is the same as the above instance call

>>>

class method, class: __main__.MyClass, val1: Value 1, the value of val2 cannot be accessed

>>> mc.val1 = 'Value changed' # Change the value of instance variable val1

>>> mc.classmd() # Instance calls the class method and notices cls.val1 The value has not changed, so cls.val1 at this time is the class variable val1, not the instance variable val1

>>>

Class method, class: __main__.MyClass, val1: Value 1, val2 cannot be accessed The value of

>>> MyClass.classmd() # class directly calls the class method, the result is the same as the above instance call

>>>

class method, class: __main__.MyClass, val1: Value 1, cannot access the value of val2

>>> MyClass.val1 = 'Class Value changed' # Change the value of the class variable val1

>>> mc.classmd() # Example Call the class method and notice that the value of cls.val1 has changed, so this further proves that cls.val1 at this time is the class variable val1, not the instance variable val1

>>>

Class method, class: __main__.MyClass, val1: Class Value changed, the value of val2 cannot be accessed

>>> MyClass.classmd() # The class directly calls the class method, the result is the same as the above instance call

>

Static method: It cannot access class attributes and instance attributes. It is equivalent to a relatively independent method and has nothing to do with the class. From another perspective, it is just a function placed in the scope of a class.

Class member methods: Class attributes can be accessed, but instance attributes cannot be accessed. The above variable val1 is a class variable in the class and an instance variable in the instance, so it is easy to confuse.