None assigment

[Steve Holden]

Simon Brunning" <SBrunning at trisystems.co.uk> wrote in message
news:mailman.981653763.25307.python-list at python.org...
> > From: Gregoire Welraeds [SMTP:greg at perceval.be]
> > > After doing "None = 2", you can "del None" to get the default value
> > > back.
>
> Hmm. This is true - I didn't know that.
>
> > If we follow that logic, I could use any non assigned variable to have
the
> > following working:
> >
> > >>> a= [1,'',3]
> > >>> filter(b,a)
> >
> > but this won't work as the interpreter complains that there is no
variable
> > named b.
>
> You can't do this with *ordinary* labels - by del'ing them, you are
> un-assigning them. But the 'None' label is *extraordinary*, obviously.
>
Oh, no it isn't. It's just defined in the __builtins__ namespace. You can't
delete this built in version, but you can, with enough effort, redefine it:

>>> None = 22
>>> print None
22
>>> del None
>>> print None
None
>>> __builtins__.None = 42
>>> print None
42
>>> del None
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: There is no variable named 'None'
>>>


> > It seems that (the so called variable) None is neither a variable like
any
> > other, nor is it a label as stated by Simon Brunning in another post.
>
> It *is* a label - try this:
>
> >>>print None
> None
> >>> None = 'spam'
> >>> print None
> spam
>
> So, clearly, None is a label.
>
Yes, but here you've added another definition of None in the module
namespace, not redefined the one you had in __builtins__ to start with.

> Now that I look at this problem, I'm totally confused about None.
> Any
> > explanaition is welcome. Anyway, I don't understand that one can
override
> > None. Could you give me at least one single good reason to do that.
>
> I can give you a good reason *not* to - it will confuse the hell out of
> people. ;-)
>
> I'm sure one of the Python gurus will give you a bit more background, but
I
> think that the short answer is this - leave 'None' alone to get on with
its
> job, and you won't have any problems.
>
On this we can, I hope, all agree.

regards
 Steve