Comment on PEP-0322: Reverse Iteration Methods

[Alex Martelli]

Raymond Hettinger wrote:

>> And I still think you don't need it often enough to put it in the
>> builtin namespace, so the function should go in the itertools module.
> 
> If you have a magic method, __riter__, then the corresponding
> function needs to be a builtin.  They go hand in hand.  The
> core parts of the language need to be usable without having
> to know about itertools.

I have already seen module copy presented as a counter-example to
your assertion, and I'd like to add module pickle as a second, and
I hope decisive, counter-example.  It is, to put it simply, utterly
false that functions corresponding to magic methods "need to be
builtins": it's *perfectly* all right for such functions to live
in standard library modules.  Oh, and let's not forget module sets:
the __as_immutable__ and __as_temporarily_immutable__ magic methods,
that are used when making a set of sets, or checking for a set's
membership in another set, can be seen as yet another example (and
in this case there is no wiggling out of it by claiming that the
magicmethod/NON-builtin correspondence is a historical/legacy thing,
as the BDFL approved module sets.py, with just this usage, so very
recently).

I second the motion that function riter, with check for __riter__
and all, should live in module itertools.  Reverse iteration is a
RARE need -- far rarer than copying or even pickling -- and there
is no real reason to burden __builtins__ with a function for it.


Alex