>>> type(len)
class 'builtin_function_or_method'>            #普通的BIF
>>> type(int)
class 'type'>             #工廠函數(shù)（類對(duì)象），當(dāng)調(diào)用它們的時(shí)候，其實(shí)就是創(chuàng)建了一個(gè)相應(yīng)的實(shí)例對(duì)象
>>> type(dir)
class 'builtin_function_or_method'>
>>> type(list)
class 'type'>

>>> a = int('123')        #創(chuàng)建一個(gè)相應(yīng)的實(shí)例對(duì)象a
>>> b = int('345')
>>> a + b              #python在兩個(gè)對(duì)象進(jìn)行相加操作
468

>>> class New_int(int):
    def __add__(self,other):
        return int.__sub__(self,other)
    def __sub__(self,other):
        return int.__add__(self,other)

    
>>> a = New_int(3)
>>> b = New_int(5)
>>> a + b    #兩個(gè)對(duì)象相加，觸發(fā) __add__(self,other)方法
-2
>>> a - b
8
>>>

實(shí)例2：錯(cuò)誤寫法，會(huì)造成無限遞歸
>>> class New_int(int):
    def __add__(self,other):
        return (self + other)  
    def __sub__(self,other):
        return (self - other)


>>> class New_int(int):
    def __add__(self,other):
        return (int(self) + int(other))       #將self與other強(qiáng)制轉(zhuǎn)換為整型，所以不會(huì)出現(xiàn)兩個(gè)對(duì)象相加觸發(fā)__add__()方法
    def __sub__(self,other):
        return (int(self) - int(other))

    
>>> a = New_int(3)
>>> b = New_int(5)
>>> a + b
8

>>> class int(int):
    def __add__(self,other):
        return int.__sub__(self,other)

    
>>> a = int(3)
>>> b = int(2)
>>> a + b
1

反運(yùn)算與算術(shù)運(yùn)算符的不同之處是，反運(yùn)算多了一個(gè)'r'，例如 __add__()的反運(yùn)算對(duì)應(yīng)為 __radd__()

>>> a + b

這里a是加數(shù)，b是被加數(shù)，如果a對(duì)象的__add__()方法沒有實(shí)現(xiàn)或者不支持相應(yīng)的操作，那么python就會(huì)自動(dòng)調(diào)用b的__radd__()方法

實(shí)例：

>>> class Nint(int):
    def __radd__(self,other):
        return int.__sub__(self,other)

    
>>> a = Nint(5)
>>> b = Nint(3)
>>> a + b      #由于a對(duì)象默認(rèn)有__add__()方法，所以b的__radd__()沒有執(zhí)行
8

實(shí)例2：

>>> class Nint(int):
    def __radd__(self,other):
        return int.__sub__(self,other)

    
>>> b = Nint(5)
>>> 3 + b         #由于3無__add__()方法，所以執(zhí)行b的反運(yùn)算__radd__(self,other)方法，其中self是b對(duì)象
2