[Python-3000] None in Comparisons

[M.-A. Lemburg]

On 2008-11-11 18:09, Guido van Rossum wrote:
> We're not going to add the "feature" back that None compares smaller
> than everything. It's a slippery slope that ends with all operations
> involving None returning None -- I've seen a proposal made in all
> earnestness requesting that None+42 == None, None() == None, and so
> on. This Nonesense was wisely rejected; a whole slew of
> early-error-catching would have gone out of the window.

I was suggesting None of that.

> It's the same
> with making None smaller than everything else. For numbers, you can
> already use -inf; for other types, you'll have to invent your own
> Smallest if you need it.

No, that doesn't work: -inf is a perfectly valid number, None isn't.

Same for strings: '' would be a valid string that compares smaller
than all others, None isn't.

Furthermore, if you get a None value from some database or data set,
you don't want to replace that special n/a value with a valid number
or string - since you'd lose round-trip safety. In some cases you
don't even know whether the item was supposed to be a number or e.g.
a string, so there is no obvious choice for a replacement.

Looks like data processing algorithms written for Python3 will have
to start using key functions throughout or end up requiring lots of
if-elif-elses to deal gracefully with the different combinations
of comparison failures.

Oh well, another surprise to add to the Python 3k list of surprises.

And here's another one:

>>> None < None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: NoneType() < NoneType()
>>> None is None
True
>>> None == None
True
>>> None > None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: NoneType() > NoneType()
>>> None != None
False

Two values that compare equal to each other (and are in fact identical),
yet cannot be compared less-than or greater-than.

This would make sense if you think of None as meaning "anything
and/or nothing", since the left side None could stand for
a different None than the right one, but then you could apply the
same logic to inf:

>>> inf = float('inf')
>>> inf < inf
False
>>> inf is inf
True
>>> inf == inf
True
>>> inf > inf
False
>>> inf != inf
False

In this case you don't get any errors.

Note all of this has to be seen from a user perspective, not from
a CPython implementors perspective.

-- 
Marc-Andre Lemburg
eGenix.com

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