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

[Chris Barker - NOAA Federal]

I agree that some more docs on the specialness of None (and, to a
lessor extent, True and False).

A few comments:

> None is a keyword
> ==============
>>>> None = 0
> SyntaxError: can't assign to keyword

One of the implications of this is that “None” will always be the
Singleton None object — so you can (and should) use:

Something is None

To test for None.

> The Command Line Interpreter hides None
> =================================
>>>> None

That’s a good one to highlight!

>
> None is false in a boolean context
> ==========================
>>>> bool(None)
> False

Maybe this belongs more in a discussion of “Falseyness”

> Procedures return None
> ==================
>>>> a = [3,1,2]
>>>> b = a.sort()
>>>> a, b
> ([1, 2, 3], None)

This is less about None than about the convention that mutating
methods return None. Maybe that discussion belongs elsewhere.

> Dictionary get returns None if not found
> ==============================
>>>> {}.get('dne') is None
> True

Belongs with dict docs really, and not really true — dict.get()
returns the default value, which is None be default.

> None is default return value
> =====================
>>>> def fn(): pass
> ...
>>>> fn() # No response!
>>>> print(fn()) # Here's why.
> None

Yup.

> None is used as a sentinel default value
> ==============================
> Particularly useful when default value must be determined
> in body of function.
> ---
> def insort_right(a, x, lo=0, hi=None):
>    # ...
>    if hi is None:
>        hi = len(a)
> ---

This is also a convention — and primarily applies to mutable defaults,
which you hardly ever want to assign directly.

So a good example of None being used as a sentinel, but nog really
anything special about None.

-CHB
> /