[Python-ideas] Introduce collections.Reiterable

Neil Girdhar mistersheik at gmail.com
Sat Sep 21 06:20:39 CEST 2013

I can humbly suggest why Python would deprecate the sequence protocol:
there "should be one obvious way" to answer iter(), and in my opinion
that's the  __iter__()  method.  I considered infinite iterators, and if
you happen to have  __getitem__ written, you can trivially write an
__iter__ function as follows:

def __iter__(self):
     return (self.__getitem__(x) for x in itertools.count())

Now your class will be Iterable in the abc sense, and no longer relies on
the sequence protocol


On Fri, Sep 20, 2013 at 10:41 PM, Stephen J. Turnbull <stephen at xemacs.org>wrote:

> Tim Delaney writes:
>  > Also, pathological is probably not the best term to use. Instead,
>  > substitute "deliberately breaks a well-established protocol".
> Note that in Neil's use case (the OP) it's not deliberate.  His
> function receives an iterable, it naively iterates it and (if an
> iterator) consumes it, and then some other function loses.  Silently.
> Also, as long as __getitem__(0) succeeds, this *is* the "sequence
> protocol".  (A Sequence also has a __len__() method, but iterability
> doesn't depend on that.)
> I don't see why Python would deprecate this.  For example, consider
> the sequence of factors of integers: [(1,2), (1,3), (1,2,2,4), (1,5),
> (1,2,3,6), ...].  Factorization being in general a fairly expensive
> operation, you might want to define this in terms of __getitem__() but
> __len__() is infinite.  I admit this is a somewhat artificial example
> (I don't know of non-academic applications for this sequence, although
> factorization itself is very useful in applications like crypto).
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