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

[David Mertz]

Can't we achieve the same effect with code like the following.  Maybe
slightly less elegant, but only slightly and hence not worth adding to the
language.

class Continuation(object):

    def __init__(self, val):

        self.val = val

# Other stuff, including creating the iteratable

x = None

while True:

    if isinstance(x, Continuation):

        x = x.val

    else:

        try:

            x = it.next()

        except StopIteration:

            break

    if something(x):

        do_things(x)

    else:

        x = Continuation(special_value)

        continue


On Wed, Sep 24, 2014 at 3:09 PM, 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.
>
> 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
> Phone: +353876363185
> Blog: http://indiebiotech.com
> miniLock.io: JjmYYngs7akLZUjkvFkuYdsZ3PyPHSZRBKNm6qTYKZfAM
>
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