Python

Class Attributes

前面都只有創建綁定在物件上的屬性
但其實class可以創建綁定class的屬性
這樣整個class都共用一個屬性
不會因為不同的物件而改變

Example

class Person:
    number_of_people = 0 
    # An attribute for the whole class
    # not specific to the objects
 
    def __init__(self, name):
        self.name = name
        Person.number_of_people += 1
 
p1 = Person("Tim")
p2 = Person("Joe")
 
print(Person.number_of_people) #2
print(p1.number_of_people) #2

@classmethod

有時你會需要class的函式操作的東西是class attributes
且不想要透過創造物件來執行
這種時候就可以使用 @classmethod

使用方式：寫在你不想用物件呼叫的函式上方
這時引數self會變成cls，class的意思

class student:
    school = "HSNU"
    
    @classmethod
    def get_school(cls):
        return cls.school

Example

# @classmethod
class Person:
    number_of_people = 0 
    GRAVITY = -9.8
    # Attributes for the whole class, not specific to the objects
    
    def __init__(self, name, age):
        self.name = name
        self.age = age
        # Person.number_of_people += 1
        Person.add_person()
 
    @classmethod # wrote on the function so that it can be called without creating an object
    def get_number_of_people(cls): 
        return cls.number_of_people
        # The things returned cannot be the attributes of the objects
        # Otherwise there will be a mistake
    @classmethod
    def add_person(cls):
        cls.number_of_people += 1
        
    '''# Like this. It's incorrect
    @classmethod
    def get_name(self):
        return self.name '''
 
p1 = Person("Tim", 30)
p2 = Person("Joe", 15)
print(Person.get_number_of_people()) #2

@staticmethod

有時候，class只是用來整理function
例如你把一堆數學相關的function
丟進一個Math的class
這種時候你根本不想要呼叫物件來執行function
只想單純呼叫
這種時候就可以用@staticmethod

使用方式：寫在該函式上方

# @staticmethod
 
class Math:
 
    @staticmethod
    def add_five(x):  # It's just a function defined in the "Math" class
        return x+5
    
    @staticmethod
    def pr():
        print("run")
 
print(Math.add_five(5))