[Python-ideas] PEP draft: context variables

[Nick Coghlan]

On 15 October 2017 at 15:05, Guido van Rossum <guido at python.org> wrote:

> I would like to reboot this discussion (again). It feels to me we're
> getting farther and farther from solving any of the problems we might solve.
>
> I think we need to give up on doing anything about generators; the use
> cases point in too many conflicting directions. So we should keep the
> semantics there, and if you don't want your numeric or decimal context to
> leak out of a generator, don't put `yield` inside `with`. (Yury and Stefan
> have both remarked that this is not a problem in practice, given that there
> are no bug reports or StackOverflow questions about this topic.)
>

Let me have another go at building up the PEP 550 generator argument from
first principles.

The behaviour that PEP 550 says *shouldn't* change is the semantic
equivalence of the following code:

    # Iterator form
    class ResultsIterator:
        def __init__(self, data):
            self._itr = iter(data)
        def __next__(self):
            return calculate_result(next(self._itr))

    results = _ResultsIterator(data)

    # Generator form
    def _results_gen(data):
        for item in data:
            yield calculate_result(item)

    results = _results_gen(data)

This *had* been non-controversial until recently, and I still don't
understand why folks suddenly decided we should bring it into question by
proposing that generators should start implicitly capturing state at
creation time just because it's technically possible for them to do so (yes
we can implicitly change the way all generators work, but no, we can't
implicitly change the way all *iterators* work).

The behaviour that PEP 550 thinks *should* change is for the following code
to become roughly semantically equivalent, given the constraint that the
context manager involved either doesn't manipulate any shared state at all
(already supported), or else only manipulates context variables (the new
part that PEP 550 adds):

    # Iterator form
    class ResultsIterator:
        def __init__(self, data):
            self._itr = iter(data)
        def __next__(self):
            with adjusted_context():
                return calculate_result(next(self._itr))

    results = _ResultsIterator(data)

    # Generator form
    def _results_gen(data):
        for item in data:
            with adjusted_context():
                yield calculate_result(item)

    results = _results_gen(data)

Today, while these two forms look like they *should* be comparable, they're
not especially close to being semantically equivalent, as there's no
mechanism that allows for implicit context reversion at the yield point in
the generator form.

While I think PEP 550 would still be usable without fixing this
discrepancy, I'd be thoroughly disappointed if the only reason we decided
not to do it was because we couldn't clearly articulate the difference in
reasoning between:

* "Generators currently have no way to reasonably express the equivalent of
having a context-dependent return statement inside a with statement in a
__next__ method implementation, so let's define one" (aka "context variable
changes shouldn't leak out of generators, as that will make them *more*
like explicit iterator __next__ methods"); and
* "Generator functions should otherwise continue to be unsurprising
syntactic sugar for objects that implement the regular iterator protocol"
(aka "generators shouldn't implicitly capture their creation context, as
that would make them *less* like explicit iterator __init__ methods").

Cheers,
Nick.

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