Why generate POP_TOP after an "import from?"

[Chris Angelico]

On Sun, Apr 19, 2020 at 7:36 AM Adam Preble <adam.preble at gmail.com> wrote:
>
> On Friday, April 17, 2020 at 1:37:18 PM UTC-5, Chris Angelico wrote:
> > The level is used for package-relative imports, and will basically be
> > the number of leading dots (eg "from ...spam import x" will have a
> > level of 3). You're absolutely right with your analysis, with one
> > small clarification:
>
> Thanks for taking that on too. I haven't set up module hierarchy yet so I'm not in a position to handle levels, but I have started parsing them and generating the opcodes. Is it sufficient to just use the number of dots as an indication of level?
>

Correct. You can literally just put that exact line of code into a
function, disassemble it, and you'll see the level - even if you're
not in a package (which would be a run-time error only).

> As a side note, I suppose it's sufficient to just *peek* at the stack rather than pop the module and push it again. I'm guessing that's what the Python interpreter is doing.
>

Yep!

> > In theory, I suppose, you could replace the POP_TOP with a STORE_FAST
> > into "sys", and thus get a two-way import that both grabs the module
> > and also grabs something out of it. Not very often wanted, but could
> > be done if you fiddle with the bytecode.
>
> I'm trying to follow along for academic purposes. I'm guessing you mean that would basically optimize:
>
> from sys import path
> import sys
>
> It would definitely be a fringe thing to do...

Exactly, an incredibly fringe thing to do :) But that's the effect
you'd get (at least, I believe so - haven't actually tested that).

ChrisA