Chapter 10 Object Oriented Programming

1. Class and instance

2. Attributes

3. Instance method and static method

10.1 Basic concept

想定義「黃色方形」物件類別：

• 這類物件有什麼共通屬性：黃色

• 各別物件有什麼基本屬性：height, width, center.

• 對它有什麼想運用的操作：算它的面積

class YellowRectangle:
    """ A Python object that describes the properties of a yellow rectangle """
    color="yellow" 
    
    def __init__(self, width, height):
        self.width = width
        self.height = height
    
    def __repr__(self):
        return "Rectangle(width={w}, height={h})".format(h=self.height, w=self.width)
        
    def compute_area(self):
        return self.width * self.height

10.2 Class and Instance

判斷class或instance:

YR=YellowRectangle # YR is a class object
yr1=YR(5,10) # yr1 is an instance
yr2=YellowRectangle(1,3)

YR is YellowRectangle # is: 判斷是否為某class object

## True

isinstance(YR,YellowRectangle) # isinstance: 判斷是否為某class下的instance

## False

isinstance(yr1,YellowRectangle)

## True

isinstance(yr2,YR)

## True

10.3 Class attributes and instance attributes

• 透過def __init__(self,...)定義的attribute為instance attributes.

• 其他def __init__(self,...)以外定義的object都會成為class attributes.

  class attribute自然會是instance attribute

以下何者為class attr？何者為instance attr?

yr1.color
yr1.height
yr1.width

在一個父姓社會，定義linFamily這個class，每個instance是一個人：

• 所有的instance一定姓“林”：即self.surname="林"

• 每個instance有自己的名：即self.firstname因人而異

• 每個instance有自己的性別：即self.gender因人而異

• 每個instance print時會顯示“我叫XXX, 性別Y”

10.4 Instance and static method

• Instance method: def ...(self,...): 第一個arg為self. self代表了instance本身。

An instance method is defined whenever a function definition is specified within the body of a class. This may seem trivial but there is still a significant nuance that must be cleared up, which is that ‘self’ is the defacto first-argument for any instance method.

When you call an instance method (e.g. func) from an instance object (e.g. inst), Python automatically passes that instance object as the first argument, in addition to any other arguments that were passed in by the user.

因此每個class body裡的def最好都加上self成第一個input。

# instance method
def functionName(self,...): 

• static method: def ...(....): 沒有用self（或cls，以後會講class method）在arg裡。

A static method is simply a method whose arguments must all be passed explicitly by the user. That is, Python doesn’t pass anything to a static method automatically.

A static method is simply a method whose arguments must all be passed explicitly by the user. That is, Python doesn’t pass anything to a static method automatically. The built-in decorator @staticmethod is used to distinguish a method as being static rather than an instance method.

@staticmethod
def functionName(...):

class Dummy:

    @staticmethod
    def static_func():
        """ A static method defined to always returns
            the string `'hi'`"""
        return 'hi'

inst=Dummy()
inst.static_func()

## 'hi'

在Basic concept範例裡，指出當中的instance method/static method。

class H5rectangle:
    height=5 # 所有H5rectangle族群高都是5
    def __init__(self,width): #旅群內的寬每個人都不同，有自己self的寬
        self.width=width
    def area(self): # 每個人都可以用area method來算自己的面積
        return self.width*self.height

延續linFamily class。

• 每個林家人有marry method，即instance.marry(spouse)。此method會增加instance.spouse屬性。

child1=linFamily("小明","男")

wife=wife={"surname":  "陳", "familyname": "小美", "gender": "女"}

child1.marry(wife)
child1.spouse

## {'surname': '陳', 'familyname': '小美', 'gender': '女'}

10.5 class method

• cls as the first argument

• declare @classmethod

class Dummy:

    @classmethod
    def class_func(cls):
        """ A class method defined to simply
            return `cls` unchanged"""
        return cls