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Class Attributes and Instance Attributes

发布于 2020-12-08 01:48:14

I am trying to learn the difference between the instance attributes and class attributes and attributes. I have the code below and I am trying to distinguish these factors.

class Student:
    firstname = ""
    lastname = ""
    ucid = ""
    department = ""
    nationality = ""
    courses = {}

    def __init__(self, fname, lname, ucid, dept, n):
        self.firstname = fname
        self.lastname = lname
        self.ucid = ucid
        self.department = dept
        self.nationality = n
        self.courses = {}

    def setName(self, fname, lname):
        self.firstname = fname
        self.lastname = lname

    def setDepartment(self, d):
        self.department = d

    def setUcid(self, u):
        self.ucid = u

    def setNationality(self, n):
        self.nationality = n

    def addCourse(self, coursename, gpa):
        self.courses[coursename] = gpa

    def printAll(self):
        print("The name of the student is ", self.firstname, self.lastname)
        print("nationality and UCID: ", self.nationality, self.ucid)
        print("Department: ", self.department)
        print("Result: ")
        for key in self.courses.keys():
            print(key, self.courses[key])

        print("--------------------\n")

s1=Student("Beth","Bean","30303","Computer Science","International")
s1.addCourse("SCIENCE",3.75)
s1.printAll()
s2=Student("Mac","Miller","30303","Envr Science","American")
s2.addCourse("MATH",4.00)
s2.printAll()

From what I understood the attributes would be: firstname,lastname,ucid,department,nationality,courses But I do not know what instance attributes and class attributes would be.

Questioner
haribo11
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tomy0608 2020-12-08 13:06:53

I am trying to learn the difference between the instance attributes and class attributes and attributes.

there should be two attributes, class attribute, instance attribute. or instance attribute&none-instance attribute for convenience.

instance attribute

  • these are things activated only when __init__ has been called.
  • you can only access thenm after Class is initialized, which commonly seen as self.xxx.
  • and methods in class with self as its first parameter(normally), these functions are instance methods, and you can only access after you initialized the Class.
  • and methods in class with @property deco, they are instance attributes
common seen instance attribute 

class Name(object):
   def __init__(self):
        self.age = 100
   def func(self):
       pass
   @property
   def age(self):
       return self.age

class attribute

non-instance attribute or static attribute, whatever you call it

  • these things stay activated along with Class.
  • which means you can access them whenever you need to, like __init__, even in __new__.
  • they can be called by both Class and instance.
common seen class attribute 

class Name(object):
   attr = 'Im class attribute'

there is something else you may should know, class method, which stay activated along with Class but the difference is class method can't be called by instance but only Class. example here

class Name(object)
   attr = 'Im class attribute'
   @classmethod
   def get_attr(cls):
       return cls.attr

Conclusion

"class attribute" can be called by both instance and Class

"instance attribute" can only called by instance.

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