Where is the NoneType located?

Question

In Python 3, I would like to check whether value is either string or None.

One way to do this is

assert type(value) in { str, NoneType }

But where is NoneType located in Python?

Without any import, using NoneType produces NameError: name 'NoneType' is not defined.

You should also use `isinstance`. It can take a tuple of types to check. `isinstance(value, (str, type(None)))` — Silas Ray, Feb 11 '14 at 15:54
If this is not for unit testing, one could argue that you usually shouldn't type check at all. — Brave Sir Robin, Feb 11 '14 at 15:57
@rmartinjak: some programmers still like to validate their coding assumptions with an `assert` statement. The support is there. *I* wouldn't use assertions and would use unit testing too, but if you are using `assert` anyway.. — Martijn Pieters, Feb 11 '14 at 16:00
Sorry if my question was put wrong, I know about `isinstance(object, class-or-type-or-tuple)`, I just wanted to focus on the problem of `NoneType`'s location. — Tregoreg, Feb 11 '14 at 16:00

Martijn Pieters · Answer 1 · 2022-04-20T13:31:39.393

You can use type(None) to get the type object, but you want to use isinstance() here, not type() in {...}:

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

The NoneType object is not otherwise exposed anywhere in Python versions older than 3.10^*.

I'd not use type checking for that at all really, I'd use:

assert value is None or isinstance(value, str)

as None is a singleton (very much on purpose) and NoneType explicitly forbids subclassing anyway:

>>> 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, to be consistent with the other singleton types being added to the types module.

bad_coder · Accepted Answer · 2020-10-11T22:39:28.947

types.NoneType is being reintroduced in Python 3.10.

What’s New In Python 3.10

Improved Modules

types

Reintroduced the types.EllipsisType, types.NoneType and types.NotImplementedType classes, providing a new set of types readily interpretable by type checkers. (Contributed by Bas van Beek in bpo-41810.)

The discussion about the change was motivated by a need for types.EllipsisType, leading to types.NoneType also being added for consistency.

score 0 · Answer 3 · edited Apr 15 '19 at 07:50

Please use type(None). You can use python shell to check like in the below function in which I use type(None) in order to change from None to NoneType.

def to_unicode(value):
'''change value to unicode'''
try:
    if isinstance(value, (str,type(None))):
        return value
    if not isinstance(value, bytes):
        raise TypeError("Expected bytes, unicode, or None; got %r" % type(value))
    return value.decode("utf-8")
except UnicodeDecodeError:
    return repr(value)

Where is the NoneType located?

3 Answers3

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