[Python-Dev] requirements for moving __import__ over to importlib?

[PJ Eby]

On Tue, Feb 7, 2012 at 3:07 PM, Brett Cannon <brett at python.org> wrote:

> So, if there is going to be some baseline performance target I need to hit
> to make people happy I would prefer to know what that (real-world)
> benchmark is and what the performance target is going to be on a non-debug
> build. And if people are not worried about the performance then I'm happy
> with that as well. =)
>

One thing I'm a bit worried about is repeated imports, especially ones that
are inside frequently-called functions.  In today's versions of Python,
this is a performance win for "command-line tool platform" systems like
Mercurial and PEAK, where you want to delay importing as long as possible,
in case the code that needs the import is never called at all...  but, if
it *is* used, you may still need to use it a lot of times.

When writing that kind of code, I usually just unconditionally import
inside the function, because the C code check for an already-imported
module is faster than the Python "if" statement I'd have to clutter up my
otherwise-clean function with.

So, in addition to the things other people have mentioned as performance
targets, I'd like to keep the slowdown factor low for this type of scenario
as well.  Specifically, the slowdown shouldn't be so much as to motivate
lazy importers like Mercurial and PEAK to need to rewrite in-function
imports to do the already-imported check ourselves.  ;-)

(Disclaimer: I haven't actually seen Mercurial's delayed/dynamic import
code, so I can't say for 100% sure if they'd be affected the same way.)
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