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

[David Mertz]

There are a couple minor errors in Nathaniels (presumably untested) code.
But I think it looks quite elegant overall, actually:

#!/usr/bin/env python3
from string import ascii_lowercase
from random import random

class PushbackAdaptor(object):
    def __init__(self, iterable):
        self.base = iter(iterable)
        self.stack = []

    def __next__(self):
        if self.stack:
            return self.stack.pop()
        else:
            return next(self.base)

    def pushback(self, obj):
        self.stack.append(obj)

    def __iter__(self):
        return self

def repeat_some(it):
    it = PushbackAdaptor(it)
    for x in it:
        print(x, end='')
        if random() > 0.5:
            it.pushback(x)
            continue
    print()

repeat_some(ascii_lowercase)
repeat_some(range(10))


On Wed, Sep 24, 2014 at 6:50 PM, Nathaniel Smith <njs at pobox.com> wrote:

> On 25 Sep 2014 02:09, "Andrew Barnert" <abarnert at yahoo.com.dmarc.invalid>
> wrote:
> >
> > On Sep 24, 2014, at 15:09, Cathal Garvey <cathalgarvey at cathalgarvey.me>
> wrote:
> >
> > >
> > > 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.
>
> The way I would describe this is, the proposal is to add single-item
> pushback support to all for loops. Tokenizers are a common case that needs
> pushback ("if we are in the IDENTIFIER state and the next character is not
> alphanumeric, then set state to NEW_TOKEN and process it again").
>
> I don't know how common such cases are in the grand scheme of things, but
> they are somewhat cumbersome to handle when they come up.
>
> The most elegant solution I know is:
>
> class PushbackAdaptor:
>     def __init__(self, iterable):
>         self.base = iter(iterable)
>         self.stack = []
>
>     def next(self):
>         if self.stack:
>             return self.stack.pop()
>         else:
>             return self.base.next()
>
>     def pushback(self, obj):
>         self.stack.append(obj)
>
> it = iter(character_source)
> for char in it:
>     ...
>     if state is IDENTIFIER and char not in IDENT_CHARS:
>         state = NEW_TOKEN
>         it.push_back(char)
>         continue
>     ...
>
> In modern python, I think the natural meaning for 'continue with' wouldn't
> be to special-case something like this. Instead, where 'continue' triggers
> a call to 'it.next()', I'd expect 'continue with x' to trigger a call to
> 'it.send(x)'. I suspect this might enable some nice idioms in coroutiney
> code, though I'm not very familiar with such.
>
> -n
>
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