Peek inside iterator (is there a PEP about this?)

Luis Zarrabeitia kyrie at
Wed Oct 1 16:46:33 CEST 2008

Hi there.

For most use cases I think about, the iterator protocol is more than enough. 
However, on a few cases, I've needed some ugly hacks.

Ex 1:

a = iter([1,2,3,4,5]) # assume you got the iterator from a function and
b = iter([1,2,3])     # these two are just examples.



has a different side effect from


After the excecution, in the first case, iterator a contains just [5], on the 
second, it contains [4,5]. I think the second one is correct (the 5 was never 
used, after all). I tried to implement my 'own' zip, but there is no way to 
know the length of the iterator (obviously), and there is also no way 
to 'rewind' a value after calling 'next'.

Ex 2:

Will this iterator yield any value? Like with most iterables, a construct

if iterator:
   # do something

would be a very convenient thing to have, instead of wrapping a 'next' call on 
a try...except and consuming the first item.

Ex 3:

if any(iterator):
   # do something ... but the first true value was already consumed and 
   # cannot be reused. "Any" cannot peek inside the iterator without 
   # consuming the value.


i1, i2 = tee(iterator)
if any(i1):
   # do something with i2


Has there been any PEP regarding the problem of 'peeking' inside an iterator? 
Knowing if the iteration will end or not, and/or accessing the next value, 
without consuming it? Is there any (simple, elegant) way around it?


Luis Zarrabeitia (aka Kyrie)
Fac. de Matemática y Computación, UH.

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