gmpy and counting None

[Robert Kern]

Mensanator wrote:
> On Oct 13, 5:16 pm, Robert Kern <robert.k... at gmail.com> wrote:
>> Mensanator wrote:
>>> On Oct 13, 2:43 pm, <mma... at gmx.net> wrote:
>>>> Hi,
>>>> I just stumbled upon the following issue (I am running Debian):
>>>> $ python
>>>> Python 2.5.2 (r252:60911, Sep 29 2008, 21:15:13)
>>>> [GCC 4.3.2] on linux2
>>>> Type "help", "copyright", "credits" or "license" for more information.>>> [2, None].count(None)
>>>> 1
>>>>>>> from gmpy import mpz
>>>>>>> [mpz(2), None].count(None)
>>>> Traceback (most recent call last):
>>>>   File "<stdin>", line 1, in <module>
>>>> TypeError: coercion to gmpy.mpz type failed
>>>> Is this a bug in gmpy?
>>> Does the underlying GMP code support Nulls?
>> I don't think it has to. Probably, it just should implement __ne__ to return
>> False if it cannot coerce. Of course, the codebase is relatively old, so it may
>> still be using __cmp__ and __coerce__. That would make things more difficult.
> 
> Ok, assuming you CAN fix it, the next question is SHOULD you
> fix it? Afetr all, mpz(None) will still raise an exception and
> wouldn't the behaviour then be inconsistent?

I don't think so. There is no particular reason that an implementation of __eq__ 
or __ne__ *must* coerce its arguments and raise an exception if it can't. There 
is certainly a case to be made for __lt__ and the rest, because they imply an 
actual numerical comparison, but __eq__ and __ne__ are used for more than 
numerical comparison in Python. One of the great things about moving away from 
__cmp__ to the multiple comparison methods is that the two concepts were no 
longer entangled.

> When I complained that sum([]) should return None instead of 0,
> the general consensus was that the 0 choice is what most people
> expect most of the time and although there are cases where None
> is the logical choice, the 0 case is NOT a bug, "this behaviour
> is by design" and I'll just have to live with it and not use
> sum() if I expect the list may be empty.
> 
> I would expect that you could make the same argument with respect
> to trying to compare an mpz to None.

That it was by design? Possible, but I have no evidence of such. Do you? If that 
is not what you meant, then I have no idea what argument you think applies to 
both cases.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco