python中的None

The sole value of the type NoneType. None is frequently used to represent the absence of a value, as when default arguments are not passed to a function. Assignments to None are illegal and raise a SyntaxError.

Python中有一种NoneType的对象类型，None是这个类型的唯一值。也就是说，None是一个对象，其类型为NoneType，作为一个对象其布尔值为False。