# go/python-tips/058

## Instance Attribute v.s. Class Attribute
實體屬性(Instance Attribute) -> color
類別屬性(Class Attribute) -> door

class Cars:
    door = 4
    def __init__(self):
        self.color = "blue"


mazda = Cars()
toyota = Cars()
print(f"mazda.door = {mazda.door}, mazda.color = {mazda.color}")
print(f"toyota.door = {toyota.door}, toyota.color = {toyota.color}")

mazda.color = "red"
Cars.door = 2

print(f"mazda.door = {mazda.door}, mazda.color = {mazda.color}")
print(f"toyota.door = {toyota.door}, toyota.color = {toyota.color}")

mazda.door  = 4, mazda.color  = blue
toyota.door = 4, toyota.color = blue

mazda.door  = 2, mazda.color  = red
toyota.door = 2, toyota.color = blue

## Problem

>>> class Cars:
...     door = 4
...     def __init__(self):
...         self.color = "blue"

>>> mazda = Cars()
>>> mazda.door = -1
>>> 
>>> mazda.door
-1

## Problems!!!

>>> class Money:
...     def __init__(self):
...         self._ratio = 30
...     def change_to_NTD(self, value):
...         return value * self._ratio

>>> US_dollar = Money()
>>> print(US_dollar.change_to_NTD(100))
3000
>>> 
>>> US_dollar._ratio = 20
>>> print(US_dollar.change_to_NTD(100))
2000

## What do we want?

>>> chiawei = People()
>>> print(f"chiawei.age = {chiawei.age}")
chiawei.age = 1


>>> chiawei.age = 98
>>> print(f"chiawei.age = {chiawei.age}")
chiawei.age = 98

>>> chiawei.age = -3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 12, in age
ValueError: I believe the age is incorrect.

## Traditional Method (Get, Set)

chiawei.get_age()
chiawei.set_age(98)

class People:
    def __init__(self, value=1):
        self.__age = value

    def get_age(self):
        return self.__age

    def set_age(self, value):
        if 0 <= value < 150:
            self.__age = value
        else:
            raise ValueError("I believe the age is incorrect.")
        
chiawei = People()
print(f"chiawei.age = {chiawei.get_age()}")

chiawei.set_age(98)
print(f"chiawei.age = {chiawei.get_age()}")

chiawei.set_age(-3)
print(f"chiawei.age = {chiawei.get_age()}")

## Read Only

>>> chiawei = People()
>>> print(f"chiawei.age = {chiawei.age}")
chiawei.age = 1

>>> chiawei.age = 98
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't set attribute 'age'

>>> print(f"chiawei.age = {chiawei.age}")
chiawei.age = 1

## Property

class People:
    def __init__(self, value=1):
        self.__age = value

    @property
    def age(self):
        return self.__age

    @age.setter
    def age(self, value):
        if 0 <= value < 150:
            self.__age = value
        else:
            raise ValueError("I believe the age is incorrect.")

# Reference
* [[Python教學] @property是什麼? 使用場景和用法介紹](https://www.maxlist.xyz/2019/12/25/python-property/)
* [這是什麼妖術？Python 的屬性 (property) 運作原理](https://dev.to/codemee/zhe-shi-shi-mo-yao-shu-python-de-shu-xing-property-yun-zuo-yuan-li-3fna)
* [[Python物件導向]3個必須瞭解的Python屬性觀念](https://www.learncodewithmike.com/2020/01/python-attribute.html)
* [python @property的介绍与使用](https://zhuanlan.zhihu.com/p/64487092)