如何使用 Class

跟函式定義的方式很像

class Speaker:
    pass

speaker1.name = 'Sean Wei'
speaker1.birthday = 'June'
speaker1.skills = ['Cybersecurity', 'Diving']

speaker2.name = 'William Mou'
speaker2.birthday = 'October'
speaker2.skills = ['Photography', 'Supercomputing']

看起來似乎沒有比較簡潔

正確的寫法

class Speaker:
    def __init__(self, name, birthday, skills):
        self.name = name
        self.birthday = birthday
        self.skills = skills

speaker1 = Speaker('Sean Wei', 'June',
                   ['Cybersecurity', 'Diving'])

speaker2 = Speaker('William Mou', 'October',
                   ['Photography', 'Supercomputing'])

speaker3 = Speaker('Sirius Koan', 'July',
                   ['Softball', 'Mail'])

名詞解釋

• method 方法：class 裡面的 function
• instance 實體：依照 class 做出來的物件
• attribute 屬性：class 裡面的變數

self

• class 中 method 的參數至少要有 self
• 決定要設定哪一個物件

__init__()

• class 中一個特殊的 method
• 是一個 constructor 建構子，初始化的時候用
• 使用 obj = classname() 建構時會被呼叫
• 一定要有 self 做為參數

課堂練習（一）

幫 class Speaker 實作 learn() 函式
讓講師向另一位講師學習相關技能

class Speaker:
    def __init__(self, name, birthday, skills):
        self.name = name
        self.birthday = birthday
        self.skills = skills

    def learn(self, teacher):
        print(f'{self=}, {self.skills=}')
        print(f'{teacher=}, {teacher.skills=}')
        # Write your code here

speaker1 = Speaker('Sean Wei', 'June', ['Cybersecurity', 'Diving'])
speaker2 = Speaker('William Mou', 'October', ['Photography', 'Supercomputing'])
speaker1.learn(speaker2)

有兩種身份

分別定義 Student 及 Employee

class Student:
    def __init__(self, name, age, student_id):
        self.name = name
        self.age = age
        self.student_id = student_id

    def say_hi():
        print(f'Hello, I am {name}!')

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

    def say_hi():
        print(f'Hello, I am {name}!')

抽取重複部分

核心概念：DRY（Don’t Repeat Yourself）

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def say_hi():
        print(f'Hello, I am {name}!')

class Student(Person):
    def __init__(self, name, age, student_id):
        super().__init__(name, age)
        self.student_id = student_id

class Employee(Person):
    def __init__(self, name, age, employee_id):
        super().__init__(name, age)
        self.employee_id = employee_id

什麼是 super()

• 代表父類別
• 透過 super() 執行上層函式
• 放在函式開頭、中間、結尾都可以

語法結構

class NormalClass:
    pass

class NormalClass():
    pass

class DerivedClass(BaseClass):
    pass

class DerivedClass(moduleName.BaseClass):
    pass

class DerivedClass(Base1, Base2, Base3):
    pass

對於多層繼承的 super() 呢？

class First:
    def __init__(self):
        print('Initializing first class')
        # super().__init__()  # no need

class Second(First):
    def __init__(self):
        print('Initializing second class')
        super().__init__()

class Third(Second):
    def __init__(self):
        print('Initializing third class')
        super().__init__()

foo = Third()

繼承多個類別的 super()

class Foo:
    def __init__(self):
        print('Initializing foo class')
        super().__init__()  # note here

class Bar:
    def __init__(self):
        print('Initializing bar class')
        # super().__init__()  # no need

class Test(Foo, Bar):
    def __init__(self):
        print('Initializing test class')
        super().__init__()

baz = Test()

特殊變數 __mro__

Method Resolution Order

print(Student.__mro__)

print(Third.__mro__)

print(Test.__mro__)

建立 Phone 類別

class Phone:
    pwd = '0000'

foo = Phone()
bar = Phone()

print(f'{Phone.pwd=}, {foo.pwd=}, {bar.pwd=}')

Output: Phone.pwd='0000', foo.pwd='0000', bar.pwd='0000'

修改 foo 密碼

foo.pwd = '1234'

print(f'{Phone.pwd=}, {foo.pwd=}, {bar.pwd=}')

Output: Phone.pwd='0000', foo.pwd='1234', bar.pwd='0000'

修改 Phone 密碼

Phone.pwd = '5678'

print(f'{Phone.pwd=}, {foo.pwd=}, {bar.pwd=}')

Output: Phone.pwd='5678', foo.pwd='1234', bar.pwd='5678'

查看 attribute

可以用 vars() 函式

print(vars(Phone))  # {'__module__': '__main__',
                    #  'pwd': '5678', '__dict__': ....}

print(vars(foo))    # {'pwd': '1234'}
print(vars(bar))    # {}

或是透過 __dict__ 取得

# Same as above
print(Phone.__dict__)
print(foo.__dict__)
print(bar.__dict__)

設定顏色

Phone.color = 'blue'

print(f'{Phone.color=}, {foo.color=}, {bar.color=}')

Output: Phone.color='blue', foo.color='blue', bar.color='blue'

設定價格

foo.price = 34_900

print(f'{Phone.price=}, {foo.price=}, {bar.price=}')

Output: AttributeError

直觀的寫法

class Person:
    skills = []
    # Note here

    def learn(self, skill):
        self.skills.append(skill)

sean = Person()
sean.learn('Cybersecurity')
sean.learn('Diving')
print(f'{sean.skills=}')

mou = Person()
mou.learn('Photography')
mou.learn('Supercomputing')
print(f'{mou.skills=}')

正確寫法

class Person:
    def __init__(self):
        # creates a new empty list for each person
        self.skills = []

    def learn(self, skill):
        self.skills.append(skill)

sean = Person()
sean.learn('Cybersecurity')
sean.learn('Diving')
print(f'{sean.skills=}')

mou = Person()
mou.learn('Photography')
mou.learn('Supercomputing')
print(f'{mou.skills=}')

讓我們為食物建立 class

class Food:
    def __init__(self, price, calorie):
        self.price = price      # Unit: NTD
        self.calorie = calorie  # Unit: kcal

chicken_nuggets = Food(65, 260)
french_fries = Food(65, 530)
oreo_flurry = Food(55, 360)
corn_soup = Food(40, 90)

印出價格、熱量

class Food:
    # ...

    def __str__(self):
        return 'price and calorie is xxx'  # 小練習

# 記得先重新執行 xxx = Food(x, x) 語句

print(french_fries)
# price = 65 NTD, calorie = 260 kcal

把食物混在一起

class Food:
    # ...

    def __add__(self, other):
        price = 0    # 請完成
        calorie = 0  # 請完成
        mixture = Food(price, calorie)
        return mixture

# 記得先重新執行 xxx = Food(x, x) 語句

oreo_nuggets = oreo_flurry + chicken_nuggets

print(oreo_nuggets)
# price = 120 NTD, calorie = 620 kcal

課堂練習（二）

把 class Food 實作得更完整

class Food:
    # ...

    def __mul__(self, num):
        pass  # 計算 num 份食物的價格、熱量

    def __truediv__(self, num):
        pass  # 平分給 num 個人後每份長怎樣

    def __eq__(self, other):
        pass  # 比較「💲價格」相不相同，暫時無視熱量
    
    def __gt__(self, other):
        pass  # 比較「🔥熱量」是否更高，暫時無視價格

    # (optional) 針對「💲價格」，除了 eq 相等之外，實作 ne 不相等
    # (optional) 針對「🔥熱量」，除了 gt 外，實作 lt 小於、ge、le

參考資料 / 延伸閱讀

• 歷年資芽講義：Py 2022, Py 2021
• 三分鐘學會 Class 運用
• 關於物件導向的繼承
• PyDoc: Class tutorial, Class reference
• Private Variables
• @Decorator
• @dataclass