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[Python-Dev] Looking for examples: proof that a list comp is a function

On Mon, May 14, 2018, 03:36 Chris Angelico <rosuav at gmail.com> wrote:

> Guido has stated that this parallel is desired and important:
> result = [f(x) for x in iter if g(x)]
> result = list(f(x) for x in iter if g(x))
> Obviously the genexp has to be implemented with a nested function,
> since there's no guarantee that it'll be iterated over in this way.
> With current semantics, you can easily prove that a list comp is
> implemented with a function by looking at how it interacts with other
> scopes (mainly class scope), but Tim's proposal may change that.
> So I'm looking for examples that prove that a list comp is executed
> inside an implicit function. Ideally, examples that are supported by
> language guarantees, but something that's "CPython has done it this
> way since 3.0" is important too.
> I'm aware of just two: the name lookup interaction that may be
> changing, and the fact that there's an extra line in a traceback. And
> the latter, as far as I know, is not guaranteed (and I doubt anyone
> would care if it changed). Are there any other provable points?

Related to the traceback one: the extra stack frame shows up in a debugger,
and a profiler counts the extra frame separately. The first often confuses
me because I don't immediately see which frame I'm in just by seeing the
line of code.

There are odd interactions between `yield`/`yield from` and comprehensions
that was discussed some months ago: "[Python-Dev] Tricky way of of creating
a generator via a comprehension expression". Wait, is this a continuation
of that discussion?

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