[Python-ideas] Membership of infinite iterators

Nick Coghlan ncoghlan at gmail.com
Wed Oct 18 20:42:29 EDT 2017

On 19 October 2017 at 08:34, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

> Nick Coghlan wrote:
>> since breaking up the current single level loops as nested loops would be
>> a pre-requisite for allowing these APIs to check for signals while they're
>> running while keeping the per-iteration overhead low
> Is there really much overhead? Isn't it just checking a flag?

It's checking an atomically updated flag, so it forces CPU cache
synchronisation, which means you don't want to be doing it on every
iteration of a low level loop.

However, reviewing Serhiy's PR reminded me that PyErr_CheckSignals()
already encapsulates the "Should this thread even be checking for signals
in the first place?" logic, which means the code change to make the
itertools iterators inherently interruptible with Ctrl-C is much smaller
than I thought it would be. That approach is also clearly safe from an
exception handling point of view, since all consumer loops already need to
cope with the fact that itr.__next__() may raise arbitrary exceptions
(including KeyboardInterrupt).

So that change alone already offers a notable improvement, and combining it
with a __length_hint__() implementation that keeps container constructors
from even starting to iterate would go even further towards making the
infinite iterators more user friendly.

Similar signal checking changes to the consumer loops would also be
possible, but I don't think that's an either/or decision: changing the
iterators means they'll be interruptible for any consumer, while changing
the consumers would make them interruptible for any iterator, and having
checks in both the producer & the consumer merely means that you'll be
checking for signals twice every 65k iterations, rather than once.


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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