英文文档：
说明：
1. 函数功能将一个整数转换成8进制字符串。如果传入浮点数或者字符串均会报错。
>>> a = oct(10)
>>> a
'0o12'
>>> type(a) # 返回结果类型是字符串
<class 'str'>
>>> oct(10.0) # 浮点数不能转换成8进制
Traceback (most recent call last):
File "<pyshell#3>", line 1, in <module>
oct(10.0)
TypeError: 'float' object cannot be interpreted as an integer
>>> oct('10') # 字符串不能转换成8进制
Traceback (most recent call last):
File "<pyshell#4>", line 1, in <module>
oct('10')
TypeError: 'str' object cannot be interpreted as an integer
2. 如果传入参数不是整数，则其必须是一个定义了__index__并返回整数函数的类的实例对象。
# 未定义__index__函数，不能转换
>>> class Student:
def __init__(self,name,age):
self.name = name
self.age = age
>>> a = Student('Kim',10)
>>> oct(a)
Traceback (most recent call last):
File "<pyshell#12>", line 1, in <module>
oct(a)
TypeError: 'Student' object cannot be interpreted as an integer
# 定义了__index__函数，但是返回值不是int类型，不能转换
>>> class Student:
def __init__(self,name,age):
self.name = name
self.age = age
def __index__(self):
return self.name
>>> a = Student('Kim',10)
>>> oct(a)
Traceback (most recent call last):
File "<pyshell#18>", line 1, in <module>
oct(a)
TypeError: __index__ returned non-int (type str)
# 定义了__index__函数，而且返回值是int类型，能转换
>>> class Student:
def __init__(self,name,age):
self.name = name
self.age = age
def __index__(self):
return self.age
>>> a = Student('Kim',10)
>>> oct(a)
'0o12'
