實例與類別資料

Python 物件導向程式設計

Alex Yarosh

Content Quality Analyst @ DataCamp

OOP 核心原則

繼承：

擴充既有程式的功能

多型：

建立一致的介面

封裝：

將資料與方法打包

實例層級資料

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 是實例屬性
self 綁定到某個實例

類別層級資料

由類別的所有實例共享的資料
在 class 本體中定義類別屬性

class MyClass:
    # Define a class attribute
    CLASS_ATTR_NAME = attr_value

類別內的「全域變數」

類別層級資料

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 由所有實例共享
不要用 self 來_定義_類別屬性
用 ClassName.ATTR_NAME 來_存取_類別屬性的值

類別層級資料

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)

為何使用類別屬性？

與類別相關的全域常數

屬性的最小值／最大值
常用值與常數，例如 Circle 類別的 pi
...

類別方法

方法本就「共享」：每個實例跑同一份程式碼
類別方法不能用實例層級資料

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...)

替代建構子

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

只能有一個 __init__()


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

用類別方法建立物件
用 return 傳回物件
cls(...) 會呼叫 __init__(...)

替代建構子

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)

包含員工姓名單行的文字檔

# 不用直接呼叫 Employee() 也能建立員工
emp = Employee.from_file("employee_data.txt")
type(emp)

__main__.Employee

一起來練習吧！

Python 物件導向程式設計