实例与类数据

Python 面向对象编程

Alex Yarosh

Content Quality Analyst @ DataCamp

OOP 核心原则

继承：

扩展现有代码功能

多态：

构建统一接口

封装：

将数据与方法打包

实例级数据

class Employee:
    def __init__(self, name, salary):
       self.name = name
       self.salary = salary

emp1 = Employee("Teo Mille", 50000) 
emp2 = Employee("Marta Popov", 65000)

name、salary 是"实例属性"
self 绑定到实例

类级数据

在同一类的所有实例间共享的数据
在 class 体中定义"类属性"

class MyClass:
    # Define a class attribute
    CLASS_ATTR_NAME = attr_value

类内的"全局变量"

类级数据

class Employee:
  # Define a class attribute
  MIN_SALARY = 30000    #<--- no self. 

  def __init__(self, name, salary):
      self.name = name
      # Use class name to access class attribute
      if salary >= Employee.MIN_SALARY:
        self.salary = salary
      else:
        self.salary = Employee.MIN_SALARY

MIN_SALARY 在所有实例间共享
定义类属性时不要用 self
访问类属性值请用 ClassName.ATTR_NAME

类级数据

class Employee:
  # Define a class attribute
  MIN_SALARY = 30000    

  def __init__(self, name, salary):
      self.name = name
      # Use class name to access class attribute
      if salary >= Employee.MIN_SALARY:
        self.salary = salary
      else:
        self.salary = Employee.MIN_SALARY

emp1 = Employee("TBD", 40000)
print(emp1.MIN_SALARY)

emp2 = Employee("TBD", 60000)
print(emp2.MIN_SALARY)

为何使用类属性？

与类相关的全局常量

属性的最小/最大值
常用值和常量，如 Circle 的 pi
...

类方法

方法已是"共享"的：每个实例运行同一代码
类方法不能使用实例级数据

class MyClass:

  @classmethod                         # <---use decorator to declare a class method

  def my_awesome_method(cls, args...): # <---cls argument refers to the class
    # Do stuff here
    # Can't use any instance attributes :(

MyClass.my_awesome_method(args...)

替代构造器

class Employee:
  MIN_SALARY = 30000
  def __init__(self, name, salary=30000):
      self.name = name
      if salary >= Employee.MIN_SALARY:
        self.salary = salary
      else:
        self.salary = Employee.MIN_SALARY

只能有一个 __init__()


  @classmethod
  def from_file(cls, filename):
      with open(filename, "r") as f:
          name = f.readline()
      return cls(name)

用类方法创建对象
用 return 返回对象
cls(...) 会调用 __init__(...)

替代构造器

class Employee:
  MIN_SALARY = 30000
  def __init__(self, name, salary=30000):
      self.name = name
      if salary >= Employee.MIN_SALARY:
        self.salary = salary
      else:
        self.salary = Employee.MIN_SALARY

  @classmethod
  def from_file(cls, filename):
      with open(filename, "r") as f:
          name = f.readline()
      return cls(name)

包含一行员工姓名的文本文件

# 不直接调用 Employee() 创建对象
emp = Employee.from_file("employee_data.txt")
type(emp)

__main__.Employee

Passons à la pratique !

Python 面向对象编程