python class variables and instance variables

We know that no matter the properties or methods defined in the class, they cannot be called directly outside the class. Therefore, we can completely regard the class as an independent scope (called a class namespace). , then the class attributes are actually variables defined in the class namespace (class methods are actually functions defined in the class namespace).

Depending on the location where the attribute is defined, class attributes can be subdivided into class attributes (subsequently represented by class variables) and instance attributes (subsequently represented by instance variables).

Class variables (class attributes) (Recommended learning: Python video tutorial)

Class variables refer to those defined in the class, but in Variables outside each class method. The characteristic of class variables is that all instantiated objects of a class can share the value of the class variable, that is, class variables can be used as public resources in all instantiated objects.

Note that class variables are recommended to be accessed directly using the class name, but they can also be accessed using the object name.

For example, the following code defines an Address class and defines multiple class attributes for the class:

class Address :
    detail = '广州'
    post_code = '510660'
    def info (self):
        # 尝试直接访问类变量
        #print(detail) # 报错
        # 通过类来访问类变量
        print(Address.detail) # 输出 广州
        print(Address.post_code) # 输出 510660
#创建 2 个类对象
addr1 = Address()
addr1.info()
addr2 = Address()
addr2.info()
# 修改Address类的类变量
Address.detail = '佛山'
Address.post_code = '460110'
addr1.info()
addr2.info()

In this program, the second and third lines of code define two class variables. When the info() method of the Address object is called for the first time in the program to output two class variables, the initial values ​​of the two class variables will be output. Next, the program modifies the values ​​of the two class variables through the Address class, so when the program outputs the two class variables through the info() method for the second time, the modified values ​​of the two class variables will be output.

Run the above code, you will see the following output:

广州
510660
广州
510660
佛山
460110
佛山
460110

You can see from the output that addr1 and addr2 share class variables. In other words, changing the value of the class variable will work for all instantiated objects of this class.

Instance variables refer to attributes defined in methods of a class. Its characteristic is that it only acts on the object that calls the method.

Note that instance variables can only be accessed through the object name and cannot be accessed directly through the class name.

Python allows access to class variables through objects, but the value of class variables cannot be modified through objects. Because modifying the value of a class variable through an object is not "assigning a value to the class variable", but defining a new instance variable.

For example, the following program:

class Inventory:
    # 定义两个类变量
    item = '鼠标'
    quantity = 2000
    # 定义实例方法
    def change(self, item, quantity):
        # 下面赋值语句不是对类变量赋值，而是定义新的实例变量
        self.item = item
        self.quantity = quantity
# 创建Inventory对象
iv = Inventory()
iv.change('显示器', 500)
# 访问iv的item和quantity实例变量
print(iv.item) # 显示器
print(iv.quantity) # 500
# 访问Inventory的item和quantity类变量
print(Inventory.item) # 鼠标
print(Inventory.quantity) # 2000

In the above program, the 8th and 9th lines of code assign values ​​to the item and quantity variables through examples, which looks very much like class variables. Assignments, but not, what they do is: redefine two instance variables.

In a class, instance variables and class variables can have the same name, but in this case, using the class object will not be able to call the class variable, because it will prefer the instance variable, so this is also not recommended. "Class variables use objects Name call" reason.

After calling the change() method of the Inventory object, the above program accesses the item and quantity variables of the Inventory object. Since the object itself has these two instance variables, the program will output the instance variables of the object. value; next, the program accesses its item and quantity class variables through Inventory. At this time, the class variables are actually accessed.

Run the above program and you will see the following output:

显示器
500
鼠标
2000

The above is the detailed content of python class variables and instance variables. For more information, please follow other related articles on the PHP Chinese website!