Is 'self' keyword Mandatory inside the class Methods?

问题

I am python Begineer and i learned that first parameter inside the method should be contain some 'self' keyword but i found the following program runs without self keyword can you explain about this below is my code...

class Student(object):
    def __init__(self,name,age):
        self.name = name
        self.age = age

    def get_biggest_number(*age):
        result=0
        for item in age:
            if item > result:
                result= item
        return result


Sam = Student("Sam",18)
Peter = Student("Peter",20)
Karen = Student("Karen",22)
Mike = Student("Michael",21)

oldest= Student.get_biggest_number(Sam.age,Peter.age,Karen.age,Mike.age)
print (f"The oldest student is {oldest} years old.")

回答1:

Code you've posted has indentation errors within it, you should first indent methods and it's content, meaning that, methods are within class. On the other hand, self refers to instance, which calls specific method and gives access to the all instance data. For example

student1 = Student('name1', 20)
student2 = Student('name2', 21)
student1.some_method(arg1)

in the last call, behind the scenes student1 is passed for self parameter of the method, meaning that all student1's data is available through self argument.

What you are trying is to use staticmethod, which has no data of the instance and is aimed to logically group class related functions without explicit instance, which does not require self in method definition:

class Student:
  ...
  @staticmethod
  def get_biggest_number(*ages):
    # do the task here

On the other hand, if you would like to track all student instances and apply get_biggest_number method automatically work on them, you just have to define class variable (rather than instance variable) and on each instance __init__ append new instance to that list:

class Student:
  instances = list()  # class variable
  def __init__(self, name, age):
    # do the task
    Student.instances.append(self)  # in this case self is the newly created instance

and in get_biggest_number method you just loop through Student.instances list which will contain Student instance and you can access instance.age instance variable:

@staticmethod
def get_biggest_number():
  for student_instance in Student.instances:
    student_instance.age  # will give you age of the instance

Hope this helps.

回答2:

You shouldn't mistake classmethod with instance methods. In python you can declare a method inside a class as classmethod. This method takes a reference to the class as the first argument.

class Student(object):
    def __init__(self,name,age):
        self.name = name
        self.age = age

    def get_biggest_number(self, *age):
        result=0
        for item in age:
            if item > result:
                result= item
        return result

    @classmethod
    def get_classname(cls):
        # Has only access to class bound items
        # gets the class as argument to access the class
        return cls.__name__

    @staticmethod
    def print_foo():
        # has not a reference to class or instance
        print('foo')

回答3:

self in python refers to the instance of the class that is created. Something like this in C# and Java. However there's some differences but in short: when you don't use self as input of a method, actually you're saying that this method does not need any instance, that means this method is a static method and will never use any of class attributes.

In your example we can call get_biggest_number method with not even one instance, for example you can call this method just like this:

Student.get_biggest_number(20,30,43,32)

and the output will be 43.

来源：https://stackoverflow.com/questions/58426672/is-self-keyword-mandatory-inside-the-class-methods

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