[Python-ideas] Introduce collections.Reiterable

[Stephen J. Turnbull]

Neil Girdhar writes:

 > 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

Better yet, Python can do it for me.  That's *why* it makes sense for
iter() to accept an object with a __getitem__ method.

I wonder if it would be possible for Iterable to provide an __iter__
method at instantiation if and only if __iter__ is not defined in the
derived class and __getitem__ is.  Then

>>> class GoodIterable1(Iterable):
...  def __iter__(self):
...   return iter([])
...
>>> gi1 = GoodIterable1(Iterable)
>>> dir(gi1)
[..., __iter__, ...]
>>> class GoodIterable2(Iterable)
...  def __getitem__(self, i):
...   return [][0]
...
>>> dir(gi2)
[..., __getitem__, __iter__, ...]    # it's magic!
>>> class BadIterable(Iterable):
...  pass
...
>>> bi = BadIterable()               # ordinary mixin __iter__ wouldn't raise
                                     # but magic one does
TypeError: can't instantiate abstract class BadIterable with abstract methods __iter__
>>> 

Although I guess the ordinary mixin will raise anyway when it tries to
call __getitem__.