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[Python-Dev] requirements for moving __import__ over toimportlib?
PJ Ebypje at telecommunity.com
Wed Feb 8 03:27:04 CET 2012
On Tue, Feb 7, 2012 at 5:24 PM, Brett Cannon <brett at python.org> wrote:>> On Tue, Feb 7, 2012 at 16:51, PJ Eby <pje at telecommunity.com> wrote:>>> 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.)>>>> IOW you want the sys.modules case fast, which I will never be able to> match compared to C code since that is pure execution with no I/O.>Couldn't you just prefix the __import__ function with something like this: ... try: module = sys.modules[name] except KeyError: # slow code path(Admittedly, the import lock is still a problem; initially I thought youcould just skip it for this case, but the problem is that another threadcould be in the middle of executing the module.)-------------- next part --------------An HTML attachment was scrubbed...URL: <http://mail.python.org/pipermail/python-dev/attachments/20120207/f3c406ee/attachment.html>
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