Where is NoneType in Python 3?

[mpb]

BPO	19438
Nosy	@rhettinger, @bitdancer, @andresdelfino
PRs	[doc] Improve NoneType/NotImplementedType/ellipsis documentation #22161

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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assignee = None
closed_at = <Date 2020-10-21.18:40:05.285>
created_at = <Date 2013-10-29.20:31:31.328>
labels = ['docs']
title = 'Where is NoneType in Python 3?'
updated_at = <Date 2020-10-21.18:40:05.285>
user = 'https://bugs.python.org/mpb'

bugs.python.org fields:

activity = <Date 2020-10-21.18:40:05.285>
actor = 'adelfino'
assignee = 'docs@python'
closed = True
closed_date = <Date 2020-10-21.18:40:05.285>
closer = 'adelfino'
components = ['Documentation']
creation = <Date 2013-10-29.20:31:31.328>
creator = 'mpb'
dependencies = []
files = []
hgrepos = []
issue_num = 19438
keywords = ['patch']
message_count = 10.0
messages = ['201666', '201667', '201670', '201684', '201702', '376613', '376614', '376620', '376667', '379228']
nosy_count = 5.0
nosy_names = ['rhettinger', 'r.david.murray', 'docs@python', 'mpb', 'adelfino']
pr_nums = ['22161']
priority = 'low'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue19438'
versions = ['Python 3.3', 'Python 3.4']

[mpb]

types.NoneType seems to have disappeared in Python 3. This is probably intentional, but I cannot figure out how to test if a variable is of type NoneType in Python 3.

Specifically, I want to write:
assert type (v) in ( bytes, types.NoneType )

Yes, I could write:
assert v is None or type (v) is bytes

But the first assert statement is easier to read (IMO).

Here are links to various Python 3 documentation about None:

[1] http://docs.python.org/3/library/stdtypes.html#index-2

[2] http://docs.python.org/3/library/constants.html#None

Link [2] says: "None The sole value of the type NoneType." However, NoneType is not listed in the Python 3 documentation index. (As compared with the Python 2 index, where NoneType is listed.)

[3] http://docs.python.org/3/library/types.html

If NoneType is gone in Python 3, mention of NoneType should probably be removed from link [2]. If NoneType is present in Python 3, the docs (presumably at least one of the above links, and hopefully also the index) should tell me how to use it.

Here is another link:

[4] http://docs.python.org/3/library/stdtypes.html#bltin-type-objects

"The standard module types defines names for all standard built-in types." (Except <class 'NoneType'> ???)

None is a built-in constant. It has a type. If None's type is not considered to be a "standard built-in type", then IMO this is surprising(!!) and should be documented somewhere (for example, at link [4], and possibly elsewhere as well.)

Thanks!

[tiran]

How about:

type(v) in (bytes, type(None))

or:

   isinstance(v, (bytes, type(None))

or:

v is None or type(v) is bytes

or:

v is None or isinstance(v, bytes)

[mpb]

Of your 4 suggestions, I mentioned #3 and #4 in my post. They are less readable, IMO.

1 and 2 are nicer, but both have an "extra" set of nested parenthesis.

While I appreciate the suggestions, I submitted this as a documentation bug, because I think I should be able to find these suggestions somewhere in the Python 3 documentation, at one (or more) of the links I included in my bug report. Also, the Python 3 documentation does mention NoneType, and if NoneType is not part of Python 3, I claim this is an error in the documentation.

And then, there is my personal favorite work-around:

NoneType = type (None)    # only needed once
assert type (v) in ( bytes, NoneType )

Or (perhaps more confusingly, LOL!):

none = type (None)
assert type (v) in ( bytes, none )

isinstance is more confusing because it takes two arguments. Whenever I use it I have to think, "isinstance" vs "instanceof", which is Python, which is Java? (And I haven't used Java seriously in years!) And then I have to think about the order of the arguments (v, cls) vs (cls, v). type is just simpler than isinstance.

[bitdancer]

See http://www.python.org/dev/peps/pep-0294/ for some background on this. The unexpected thing is actually that the types module still exists at all in Python3 :)

That said, its documentation could, indeed, use some improvement to address this kind of question.

Searching the python-dev email list for 'NoneType removal types module" turns up some interesting posts, back in the 2.3 days...basically, type(None) is the One True Way to get NoneType :)

[mpb]

Regarding http://www.python.org/dev/peps/pep-0294/ ...

Complete removal of the types module makes more sense to me than letting types continue, but removing NoneType from it!

If type(None) is the one_true_way, then the docs should say that, possibly in multiple locations.

[rhettinger]

I would support adding NoneType back to the types module. Am not sure why it was ever removed. It has a much reason to exists as types.FunctionType which is a clear, well-named short-cut for "type(lambda :None)".

[andresdelfino]

types.NoneType was removed here: c9543e4#diff-0f021aec4e35b86a3160d44069dec997

The thing of adding NoneType to types that is somewhat unpleasing to me is that it's named exactly as the actual type. Seems confusing.

[rhettinger]

Thanks for the link. It looks like NoneType got inadvertently caught up in the sweep of names that had direct synonyms.

[andresdelfino]

ammar2 found this mail mentioning the changes in that commit https://mail.python.org/pipermail/python-dev/2007-November/075386.html

"I've removed the 'new' module from py3k and also removed a lot of types
from the 'types' module in py3k. It only contains types that aren't
easily available through builtins."

[andresdelfino]

As per #22336 I believe this issue can be closed now.

My PR is not relevant to the problem stated by OP, so I'm "unlinking" it.