[Python-ideas] Python docs page: In what ways is None special

[Steven D'Aprano]

On Mon, Jul 23, 2018 at 12:15:14PM +0100, Paul Moore wrote:

> The examples are interesting, agreed. One thing they highlight to me
> is that most uses of None are effectively convention. The only two
> behaviours that are unique to None and implemented in the interpreter
> are:
> 
> 1. Functions that don't return an explicit value return None.

That is built into the language.

> 2. The interactive interpreter does not display the value of a typed
> expression when that value is None.

But that is not built-in, it is merely the default behaviour. It is 
controlled by sys.displayhook, which can be customized:

https://docs.python.org/3/library/sys.html#sys.displayhook

We can have many interactive interpreters, including iPython, bpython, 
IDLE, etc and they don't necessarily suppress the printing of None.

 
> One of the key points about this proposal 

That would be the None-aware PEP, not Jonathon's proposal to add 
documentation about None.


> is that it adds a number of
> additional ways in which the interpreter treats None specially (i.e.,
> four dedicated operators). That elevates None from the position of
> being a commonly used sentinel value to one that has language support
> for being a sentinel.

None is already a special value. It is so special, it is one of only 
three built-in values given keyword status. All the other built-ins, 
including such things that are used implicitly by the interpreter (such 
as NotImplemented) are merely ordinary names.


> That's not necessarily a bad thing, but it is a
> point that should be considered when reviewing this PEP (people who
> prefer to use object() for their sentinels are probably less happy
> about making None "special",

You make it sound like its a mere aesthetic choice. None is the default, 
recommended, most convenient sentinel.

I use object() as a sentinel, when None is an otherwise ordinary value. 
I am also happy for None to be special, because it *is* special.



-- 
Steve