[Python-Dev] relative import circular problem
brett at python.org
Fri Apr 5 02:03:11 CEST 2013
On Apr 4, 2013 6:47 PM, "Guido van Rossum" <guido at python.org> wrote:
> +1 on Brett and PJE just doing this.
I'll file a bug when I get home.
> On Thu, Apr 4, 2013 at 3:38 PM, Brett Cannon <brett at python.org> wrote:
> > On Thu, Apr 4, 2013 at 5:00 PM, PJ Eby <pje at telecommunity.com> wrote:
> >> On Thu, Apr 4, 2013 at 4:42 PM, Guido van Rossum <guido at python.org>
> >> > I do think it would be fine if "from a import b" returned the
> >> > attribute 'b' of module 'a' if it exists, and otherwise look for
> >> > module 'a.b' in sys.modules.
> >> Technically, it already does that -- but inside of __import__, not in
> >> the IMPORT_FROM opcode.
> >> But then *after* doing that check-and-fallback, __import__ doesn't
> >> assign a.b, because it assumes the recursive import it called has
> >> already done this...
> > It's an unfortunate side-effect of having loaders set sys.modules for
> > modules not also set them as an attribute on their parent package
> > immediately as well (or you could argue it's a side-effect of not
> > a module instead of a name to load_module() but that's another
> >> which means that when __import__ returns, the
> >> IMPORT_FROM opcode tries and fails to do the getattr.
> >> This could be fixed in one of two ways. Either:
> >> 1. Change importlib._bootstrap._handle_fromlist() to set a.b if it
> >> successfully imports 'a.b' (inside its duplicate handling for what
> >> IMPORT_FROM does), or
> > It's three lines, one of which is 'else:'. Just did it.
> >> 2. Change the IMPORT_FROM opcode to handle the fallback itself
> >> While the latter involves a bit of C coding, it has fewer potential
> >> side-effects on the import system as a whole, and simply ensures that
> >> if "import" would succeed, then so would "from...import" targeting the
> >> same module.
> >> (There might be other fixes I haven't thought of, but really, changing
> >> IMPORT_FROM to fallback to a sys.modules check is probably by far the
> >> least-invasive way to handle it.)
> > This is my preference as well. The change would be small: I think all
> > need to do is if the getattr() fails then fall back to sys.modules.
> > if it were me and I was casting backwards-compatibility to the wind I
> > rip out the whole fromlist part of __import__() and let the bytecode
> > about the fromlist, basically making the import opcode call
> > importlib.import_module().
> --Guido van Rossum (python.org/~guido)
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