[Python-ideas] "continue with" for dynamic iterable injection

[Andrew Barnert]

On Sep 24, 2014, at 15:09, Cathal Garvey <cathalgarvey at cathalgarvey.me> wrote:

> Hello all,
> First time post, so go gentle. :)
> 
> During a twitter conversation with @simeonvisser, an idea emerged that
> he suggested I share here.
> 
> The conversation began with me complaining that I'd like a third mode of
> explicit flow control in Python for-loops; the ability to repeat a loop
> iteration in whole. The reason for this was that I was parsing data
> where a datapoint indicated the end of a preceding sub-group, so the
> datapoint was both a data structure indicator *and* data in its own
> right. So I'd like to have iterated over the line once, branching into
> the flow control management part of an if/else, and then again to branch
> into the data management part of the same if/else.
> 
> Yes, this is unnecessary and just a convenience for parsing that I'd
> like to see.

It would really help to have specific use cases, so we can look at how much the syntactic sugar helps readability vs. what we can write today. Otherwise all anyone can say is, "Well, it sounds like it might be nice, but I can't tell if it would be nice enough to be worth a language change", or try to invent their own use cases that might not be as nice as yours and then unfairly dismiss it as unnecessary.

> 
> During the discussion though, a much more versatile solution presented
> itself: repurposing the `continue` keyword to `continue with`, similarly
> to the repurposing of `yield` with `yield from`. This would avoid
> keyword bloat, but it would be explicit and intuitive to use, and would
> allow a pretty interesting extension of iteration with the for-loop.
> 
> The idea is that `continue with` would allow injection of any datatype
> or variable, which becomes the next iterated item. So, saying `continue
> with 5` means that the next *x* in a loop structured as `for x in..`
> would be 5.
> 
> This would satisfy my original, niche-y desire to re-iterate over
> something, but would also more broadly allow dynamic injection of *any*
> value into a for-loop iteration.
> 
> As an extension to the language, it would present new and exciting ways
> to iterate infinitely by accident. It would also present new and
> exciting ways for people to trip up on mutable data; one way to misuse
> this is to iterate over data, modifying the data, then iterating over it
> again. The intention, rather, is that the repeated iteration allows
> re-iteration over unaltered data in response to a mid-iteration change
> in state (i.e., in my case, parsing a token, changing parser state, and
> then parsing the token again because it also carries informational content).
> 
> However, bad data hygiene has always been part of Python, because it's a
> natural consequence of the pan-mutability that makes it such a great
> language. So, given that the same practices that an experienced Python
> dev learns to use would apply in this case, I don't think it adds
> anything that an experienced dev would trip over.
> 
> As a language extension, I've never seen something like "continue with"
> in another language. You can mimic it with recursive functions and
> `cons`-ing the injection, but as a flow-control system for an imperative
> language, nope..and Python doesn't encourage or facilitate recursion
> anyway (no TCE).
> 
> I'd love thoughts on this. It's unnecessary; but then, once something's
> declared turing complete you can accuse any additional feature of being
> unnecessary. I feel, more to the point, that it's in keeping with the
> Python philosophy and would add a novel twist to the language not seen
> elsewhere.
> 
> best,
> Cathal
> 
> -- 
> Twitter: @onetruecathal, @formabiolabs
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