课时42：魔法方法：算术运算

>>> type(len)
<class 'builtin_function_or_method'>
>>> type(int)
<class 'type'>
>>> type(dir)
<class 'builtin_function_or_method'>
>>> type(list)
<class 'type'>

>>> class C:
    pass

>>> type(C)
<class 'type'>

>>> a = int('123')
>>> b = int('345')
>>> a + b
468

__add__(self, other)                   定义加法的行为：+
__sub__(self, other)                   定义减法的行为：-
__mul__(self, other)                   定义乘法的行为：*
__truediv__(self, other)               定义真除法的行为：/
__floordiv__(self, other)              定义整数除法的行为：//
__mod__(self, other)                   定义取模算法的行为：%
__divmod__(self, other)                定义当被 divmod() 调用时的行为
__pow__(self, other[, modulo])         定义当被 power() 调用或 ** 运算时的行为
__lshift__(self, other)                定义按位左移位的行为：<<
__rshift__(self, other)                定义按位右移位的行为：>>
__and__(self, other)                   定义按位与操作的行为：&
__xor__(self, other)                   定义按位异或操作的行为：^
__or__(self, other)                    定义按位或操作的行为：|

>>> 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
-2
>>> a - b
8

>>> class Try_int(int):
    def __add__(self,other):
        return self + other
    def __sub__(self,other):
        return self - other

    
>>> a = Try_int(1)
>>> b = Try_int(3)
>>> a + b
Traceback (most recent call last):
  File "<pyshell#51>", line 1, in <module>
    a + b
  File "<pyshell#48>", line 3, in __add__
    return self + other
  File "<pyshell#48>", line 3, in __add__
    return self + other
  File "<pyshell#48>", line 3, in __add__
    return self + other
  [Previous line repeated 990 more times]
RecursionError: maximum recursion depth exceeded

def __add__(self,other):
        return self + other

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

    
>>> a = New_int(1)
>>> b = New_int(3)
>>> a + b
4

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

    
>>> a = int('5')
>>> b = int('3')
>>> a + b
2