Iterator / Iteratable confusion

[Michael Spencer]

Michael Spencer wrote:
>But, notwithstanding the docs, it is not essential that
>iter(iterator) is iterator

Terry Reedy wrote:
 > iter(iterator) is iterator is part of the iterater protocol
 >
> [...]I interpret [your post] as saying three things:
> 
> 1. "There is more than one possible definition of 'iterator'."

Terry, thanks for responding in depth.


> 2. "It is not essential to not do something wasteful as long as it is 
> otherwise inconsequential."

Not that "iter(iterator) is iterator" is somehow wasteful (actually it seems
conservative), but rather that alternative behavior is readily implmented.  You 
point out, reasonably, that if I do that, then what I get is not then an 
iterator, because it fails to conform with the protocol.

However, I suggest that there may be cases where "iter(iterator) is not 
iterator" is useful behavior.  What to call such an object is another matter.

For example, consider:

import itertools as it
def tee2(iterable):
     class itertee(object):
         def __init__(self, iterator):
             self.iterator = iterator
         def __iter__(self):
             return itertee(self.iterator.__copy__())
         def next(self):
             return self.iterator.next()
     return itertee(it.tee(iterable, 1)[0])

This returns an itertee instance which simply wraps the tee iterator returned by 
  itertools.  However iter(itertee instance) returns a copy of its iterator.  So 
this object creates as many independent iterators over iterable as are required.

In an earlier post in this thread, I included several examples of generating 
infinite series using iterator-copying like this.  I implemented the copying as 
a method of a containing iterable 'Stream', rather than of the iterators 
themselves, partly to respect the 'iterator protocol'.


> 3. "You can substitute a copy of an object that is never mutated for the 
> object itself."
> 
This was not my intended point, although I accept that my example was too abstract.

Cheers

Michael