>>> b = bin(3) 
>>> b
'0b11'
>>> type(b) #獲取b的類型
<class 'str'>

>>> class A:
 pass

>>> a = A()
>>> bin(a) 
Traceback (most recent call last):
 File "<pyshell#15>", line 1, in <module>
 bin(a)
TypeError: 'A' object cannot be interpreted as an integer

>>> class B:
 def __index__(self):
 return "3"

>>> b = B()
>>> bin(b)
Traceback (most recent call last):
 File "<pyshell#21>", line 1, in <module>
 bin(b)
TypeError: __index__ returned non-int (type str)

>>> class C:
 def __index__(self):
 return 3

>>> c = C()
>>> bin(c)
'0b11'