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instance attribute, class attribute
In object-oriented development, an instance created using a class is an object, so is the class an object?
instance attribute
Objects created through classes are called instance objects, and object attributes are also called instance attributes, which record the data of each object, and the instance attributes of different objects with the same name, the recorded data are independent and do not interfere with each other.
class Person(object):
def __init__(self,name,age):
# 这里的name和age都属于是实例属性，每个实例在创建时，都有自己的属性
self.name = name
self.age = age
Every time an object is created, the object has its own name and age properties
p1 = Person('张三',18)
p2 = Person("李四",20)
class attribute
A class attribute is an attribute owned by a class object, which is shared by all instance objects of the class, and a class attribute can be accessed through a class object or an instance object.
class Dog:
type = "狗" # 类属性
Whether it is dog1, dog2 or Dog class, you can access the type attribute
print(Dog.type) # 结果：狗
print(dog1.type) # 结果：狗
print(dog2.type) # 结果：狗
scenes to be used
A class attribute is defined when an item of data recorded by an instance of a class is always consistent.
Instance attributes require each object to open up a separate memory space for it to record data, while class attributes are shared by all classes, occupying only one memory, saving more memory space.
important point:
- Try to avoid class attributes and instance attributes with the same name. If there is an instance attribute with the same name, the instance object will first access the instance attribute.
class Dog(object):
type = "狗" # 类属性
def __init__(self):
self.type = "dog" # 对象属性
create object
dog1 = Dog()
print(dog1.type) # 结果为 “dog” 类属性和实例属性同名，使用实例对象访问的是实例属性
- Class attributes can only be modified through class objects, not instance objects
class Dog(object):
type = "狗" # 类属性
dog1 = Dog()
dog1.type = "dog" # 使用实例对象创建了对象属性type
print(dog1.type) # 结果为 “dog” 类属性和实例属性同名，访问的是实例属性
print(Dog.type) # 结果为 "狗" 访问类属性
# 只有使用类名才能修改类属性
Dog.type = "土狗"
print(Dog.type) # 土狗
dog2 = Dog()
print(dog2.type) # 土狗
- Class attributes can also be set to private by adding two underscores in front. like:
class Dog(object):
count = 0 # 公有的类属性
__type = "狗" # 私有的类属性
print(Dog.count) # 正确
print(Dog.__type) # 错误,私有属性，外部无法访问。
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