classification
Title: Where is NoneType in Python 3?
Type: Stage:
Components: Documentation Versions: Python 3.3, Python 3.4
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: docs@python Nosy List: docs@python, mpb, r.david.murray
Priority: low Keywords:

Created on 2013-10-29 20:31 by mpb, last changed 2013-11-17 13:58 by christian.heimes.

Messages (5)
msg201666 - (view) Author: mpb (mpb) Date: 2013-10-29 20:31
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!
msg201667 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2013-10-29 20:33
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)
msg201670 - (view) Author: mpb (mpb) Date: 2013-10-29 20:49
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.
msg201684 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2013-10-29 22:07
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 :)
msg201702 - (view) Author: mpb (mpb) Date: 2013-10-30 01:17
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.
History
Date User Action Args
2013-11-17 13:58:16christian.heimessetpriority: normal -> low
nosy: - christian.heimes

versions: + Python 3.4
2013-10-30 01:17:21mpbsetmessages: + msg201702
2013-10-29 22:07:06r.david.murraysetnosy: + r.david.murray
messages: + msg201684
2013-10-29 20:49:24mpbsetmessages: + msg201670
2013-10-29 20:33:51christian.heimessetnosy: + christian.heimes
messages: + msg201667
2013-10-29 20:31:31mpbcreate