---

title: Python Class（上）
image: https://i.imgur.com/hfBqYPs.jpg
tags: cpsd

---

## Class 類別

class ClassName:
    psss

- 自定義物件
- 把數個資料或函數綁在一起

### 多用class

- 包裝你的程式
- 好讀
- 好懂
- 減少重複性高的程式碼
- 物件導向程式設計（有興趣可以google這個）
- 看起來比較專業 :heavy_check_mark:

### 怎麼用

class Computer:
    pass

cp_1 = Computer()
cp_1.cpu = 'R5-4500'
cp_1.size = 16
cp_1.price = 35000

cp_2 = Computer()
cp_2.cpu = 'M1'
cp_2.size = 14
cp_2.price = 40000

# 印印看
print(cp_1)
print(cp_1.cpu)
print(cp_2.price)

- class 底下的資料 (cpu, size, price) 稱為屬性

### magic methods 魔術方法

- class 中以雙底線包住的函數稱為 magic methods，通常是系統定義的名字
- 知道自己在寫甚麼的時候才這樣命名

class Computer:
    def __init__(self, cpu, size, price):
        self.cpu = cpu
        self.size = size
        self.price = price

- 除了 `__init__`，其他魔術方法還有 `__add__`、`__sub__`、`__str__`...
- 用來告訴 python 這個類別執行某些動作（初始化、四則運算、型態轉換...）時要怎麼處理
- `__init__`定義初始化（建構物件）時的動作
- `__init__`中的參數名稱不一定要跟屬性一樣，但不一樣會導致混亂且沒必要不一樣

class Car:
    def __init__(self, x, y, z):
        self.name = x
        self.price = y
        self.color = z
# 合法但極不推薦

cp_3 = Computer('Ryzen Threadripper 3970X', 32, 120000)

- 將參數 cpu 存入屬性 cp_3.cpu
- 將參數 size 存入屬性 cp_3.size
- 將參數 price 存入屬性 cp_3.price

### method 方法

- class 底下的函數稱為 method

class Computer:
    def discount(self):
        self.price *= 0.9
    
    def size_cm(self):
        return self.size*2.54
    
    def rename_cpu(self, newName):
        self.cpu = newName
        
print(cp_3.cpu)
# output: Ryzen Threadripper 3970X
cp_3.rename_cpu('Intel Xeon E-2314')
print(cp_3.cpu)
# output: Intel Xeon E-2314

print(cp_3.size, 'inch =', cp_3.size_cm(), 'cm')
# output: 32 inch = 81.28 cm

print(cp_3.price)
# output: 120000
cp_3.discount()
print(cp_3.price)
# output: 108000.0

- 不想噴錯或發生奇怪的事情的話第一個參數一定要放 self

### Instance 實體

class 只是設計圖（樣板）
實際存在的是 instance ，依照 class 將東西做出來
class 的好處就是大量製造多個資料和功能的實體

:::success
### 小練習0

小明不小心把墨水打翻在他的 code 上，好樣的小明

class Student:
    def __init__(self, name, math, english, chinese):
        墨墨墨墨墨
        墨墨墨墨墨
        墨墨墨墨墨
        墨墨墨墨墨
        
    def average(墨墨墨墨墨):
        墨墨墨墨墨 # 回傳平均成績
        
    def is_pass(墨墨墨墨墨):
        墨墨墨墨墨 # 若且唯若每科成績都不低於 60 則回傳 True

me = Student('Sun', 101, 6, 60)
print(me.average())
print(me.is_pass())

:::
<!--解答
class Student:
def __init__(self, name, math, english, chinese):
self.name = name
self.math = math
self.english = english
self.chinese = chinese
def average(self):
return sum((self.math, self.english, self.chinese)) / 3 # 回傳三科的平均值
def is_pass(self):
return min((self.math, self.english, self.chinese)) >= 60 # 每一科都不低於60分就回傳true

boss = Student('Sophia', 1000, 100, 100)
print(boss.average())
print(boss.is_pass())
-->

:::success
### 小練習1
挑戰題

from math import gcd
class Rational:
    def __init__(self, p, q):
        t = gcd(p, q)
        p, q = p // t, q // t
        self.p = p
        self.q = q
    def __str__(self):
        return '{}/{}'.format(self.p, self.q)
    def __add__(self, a):
        # print('{} + {}'.format(self, a))
        nu = self.p * a.q + a.p * self.q
        di = self.q * a.q
        com = gcd(nu, di)
        return Rational(nu // com, di // com)

    # Your part here!!
    def __sub__(self, a): # x - y
        # print('{} - {}'.format(self, a))
        pass
    def __mul__(self, a): # x * y
        # print('{} * {}'.format(self, a))
        pass
    def __truediv__(self, a): # x / y
        # print('{} / {}'.format(self, a))
        pass
    def __eq__(self, a): # x == y
        # print('{} == {}'.format(self, a))
        pass

if __name__ == '__main__': 
    r = Rational(1, 4)
    s = Rational(3, 8)
    t = Rational(2, 8)

    print(r + s)
    print(r - s)
    print(r * s)
    print(r / s)
    print(r == s)
    print(r == t)

:::

<!--解答
from math import gcd
class Rational:
def __init__(self, p, q):
t = gcd(p, q)
p, q = p // t, q // t
self.p = p
self.q = q
def __str__(self):
return '{}/{}'.format(self.p, self.q)
def __add__(self, a):
# print('{} + {}'.format(self, a))
nu = self.p * a.q + a.p * self.q
di = self.q * a.q
com = gcd(nu, di)
return Rational(nu // com, di // com)

# Your part here!!
def __sub__(self, a): # x - y
# print('{} - {}'.format(self, a))
nu = self.p * a.q - a.p * self.q
di = self.q * a.q
return Rational(nu, di)

def __mul__(self, a): # x * y
# print('{} * {}'.format(self, a))
nu = self.p * a.p
di = self.q * a.q
return Rational(nu, di)

def __truediv__(self, a): # x / y
# print('{} * {}'.format(self, a))
nu = self.p * a.q
di = self.q * a.p
return Rational(nu, di)

def __eq__(self, a): # x == y
# print('{} == {}'.format(self, a))
return self.p == a.p and self.q == a.q

if __name__ == '__main__':
r = Rational(1, 4)
s = Rational(3, 8)
t = Rational(2, 8)

print(r + s) # Ratinal.__add__(r, s)
print(r - s)
print(r * s)
print(r / s)
print(r == s)
print(r == t)
-->

### 可以開始打專案一了~