[Python-ideas] Membership of infinite iterators

Koos Zevenhoven k7hoven at gmail.com
Tue Oct 17 13:39:14 EDT 2017

On Tue, Oct 17, 2017 at 5:26 PM, Serhiy Storchaka <storchaka at gmail.com>

> 17.10.17 17:06, Nick Coghlan пише:
>> Keep in mind we're not talking about a regular loop you can break out of
>> with Ctrl-C here - we're talking about a tight loop inside the interpreter
>> internals that leads to having to kill the whole host process just to get
>> out of it.
> And this is the root of the issue. Just let more tight loops be
> interruptible with Ctrl-C, and this will fix the more general issue.
​Not being able to interrupt something with Ctrl-C in the repl or with the
interrupt command in Jupyter notebooks is definitely a thing I sometimes
encounter. A pity I don't remember when it happens, because I usually
forget it very soon after I've restarted the kernel and continued working.
But my guess is it's usually not because of an infinite iterator.

Regarding what the OP might have been after, and just for some wild
brainstorming based on true stories: In some sense, x in y should always
have an answer, even if it may be expensive to compute. Currently, it's
possible to implement "lazy truth values" which compute the bool value
lazily when .__bool__() is called. Until you call bool(..) on it, it would
just be Maybe, and then after the call, you'd actually have True or False.
In many cases it can even be enough to know if something is Maybe true.
Also, if you do something like any(*truth_values), then you could skip the
Maybe ones on the first pass, because if you find one that's plain True,
you already have the answer.

Regarding `x in y`, where y is an infinite iterable without well defined
contents, that would return an instance of MaybeType, but .__bool__() would
raise an exception.


+ Koos Zevenhoven + http://twitter.com/k7hoven +
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