英文文档：

bin(x)
Convert an integer number to a binary string. The result is a valid Python expression. If x is not a Python int object, it has to define an__index__()
method that returns an integer.

说明：

1.将一个整形数字转换成二进制字符串

>>> b = bin(3)
>>> b
'0b11'
>>> type(b) #获取b的类型
<class 'str'>

2.如果参数x不是一个整数，则x必须定义一个 index() 方法，并且方法返回值必须是整数。

2.1 如果对象不是整数，则报错

>>> 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

2.2 如果对象定义了__index__方法，但返回值不是整数，报错

>>> 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)

2.3 对象定义了__index__方法，且返回值是整数，将__index__方法返回值转换成二进制字符串

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

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