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

Franklin? Lee leewangzhong+python at gmail.com
Mon May 14 03:52:01 EDT 2018

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?

-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20180514/02a747ff/attachment.html>

More information about the Python-Dev mailing list