[Python-ideas] Introduce collections.Reiterable

Terry Reedy tjreedy at udel.edu
Tue Sep 24 22:39:28 CEST 2013

On 9/24/2013 12:21 AM, Steven D'Aprano wrote:

> Maybe it's the mathematician in me speaking, but I don't think very many
> unbounded iterators are found outside of maths sequences.

Perhaps you are confusing 'actual infinity' or mathematics with the 
potential infinity of iterators. Unbound, or more exactly, potentially 
unbounded iterators are quite common.

First, many source iterators based on external sources are or are 
potentially unbounded. For example, text-mode files are text line 
iterators. Files based on finite disk files are bounded, but others 
(based on keyboard, socket, or other input channels) may not be. 
Consider the following example (simplified, like all examples, for 
illustrative purposes).

def source(prompt):
   "Yield user responses to prompt."
   while True:
     yield input(prompt))
   # Even if 'quit' were recognized and turned into StopIteration,
   # it still might never happen.


def measures(read_instrument):
   "Yield values returned by read_instrument."
   while True:
     yield read_instrument()

A queue can yield an unbounded sequence even if it is always finite and 
even it it has a maximum size, perhaps because the pool of potential 
queue members is finite.

Second, many transform iterators are unbounded if the input iterable is 

def transform(func, iterable):
   for item in iterable:
       yield func(item)
     except ValueError:

for i in transform(int, source('Enter an integer: ')):
   # process unbounded stream of ints.

Filter, map, and some itertools potentially produce infinite iterators. 
Itertools.islice turns infinite iterables finite.
Itertools.cycle turns finite iterables infinite.

At the highest level, interactive apps, including OSes, usually process 
indefinite streams of user-generated events.

> After all,
> even if you were to iterate over every atom in the universe, that would
> be bounded, and quite small compared to some of the numbers
> mathematicians deal with... :-)

The atoms of the universe can be reused over and over again in the same 
or different combinations to keep the iteration going indefinitely.

Terry Jan Reedy

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