NoneType 位于哪里？

在Python 3中，我想检查是否value是字符串或None.

一种方法是

assert type(value) in { str, NoneType }

但在哪里NoneType位于Python 中？

无需任何导入，使用NoneType产生NameError: name 'NoneType' is not defined.

您可以使用type(None)获取类型对象，但您想使用isinstance()在这里，不type() in {...}:

assert isinstance(value, (str, type(None)))

The NoneType object is not otherwise exposed anywhere in Python versions older than 3.10 https://github.com/python/cpython/issues/85976^*.

我实际上根本不会使用类型检查，我会使用：

assert value is None or isinstance(value, str)

as None是一个单身人士（非常有意）并且NoneType无论如何明确禁止子类化：

>>> type(None)() is None
True
>>> class NoneSubclass(type(None)):
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: type 'NoneType' is not an acceptable base type

^* As of Python 3.10, you could use types.NoneType https://docs.python.org/3/library/types.html#types.NoneType, to be consistent with the other singleton types being added to the types module https://github.com/python/cpython/issues/85976.