[Python-ideas] Introduce collections.Reiterable

[Neil Girdhar]

This is an idea I've wanted for a while:

When I call functions that accept an iterable, I often have to check 
whether the function iterates over the iterable once or more than once.  If 
it iterates more than once, I must not pass a generator, but rather cast to 
a list.  Otherwise, the second iteration through the generator will be 
empty as the first has exhausted it completely.

It would be nice to introduce an abstract base class in collections (docs<http://docs.python.org/3.3/library/collections.abc.html>) 
between Iterable and Sequence.  Right now, Sequence inherits from Iterable. 
 I propose having Sequence inherit from Reiterable, which in turn, inherits 
from Iterable.  All sequences are reiterable, whereas generators are not. 
 However, views in sets and dictionaries, and numpy arrays are examples of 
Reiterables that are not Sequences. Having such an abstract base class 
would be useful for debugging in its own right.

Also, functions that iterate twice over an iterable can check to make sure 
the iterable is "re-iterable" using isinstance (the standard approach as 
per pep 3119 <http://www.python.org/dev/peps/pep-3119/>).  But, better yet, 
itertools could add two functions: auto_tee, which takes an iterable "I" as 
its parameter, and an integer n.  If it is not a reiterable, it calls tee 
and returns n iterables independently capable of iterating "I".  If it is 
reiterable, it returns [I] * n.  This way, the client code can do whatever 
is easiest and the target code can call auto_tee if necessary.  auto_list 
could do the same sort of thing, but omitting the copy that would normally 
be incurred if a list were passed in.

Maybe this is less useful than I once thought since I've gotten by without 
it, but I just wanted to throw the idea out there in case it clicks for 
someone else.

Neil
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