I'm aware `foo(z)` gets called twice, but I wouldn't choose to break it into the `for` loop you mention:

    [bar(x) for x in map(foo, stuff) if x]

I was just trying to be as close to the original as possible.

I do find it odd you call out the calling of `foo` twice as a performance drain but then use the chief example of:

> When I'm developing out an idea and just want to see if the code works and passes my tests, I would rather have the option to insert behavior swiftly without having to reformat an entire block of code.

Tests are not something I (personally) consider to be performance sensitive. But like I said I am sort of neutral on this change. I don't really see a massive benefit to it, so I would personally air on the side of "don't add" but I'm not mad it's being added. It's a "meh" for me.