On Thu, Apr 4, 2013 at 5:00 PM, PJ Eby <p...@telecommunity.com> wrote:
> On Thu, Apr 4, 2013 at 4:42 PM, Guido van Rossum <gu...@python.org> wrote: > > 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 new 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 passing in a module instead of a name to load_module() but that's another discussion). > 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 you need to do is if the getattr() fails then fall back to sys.modules. Although if it were me and I was casting backwards-compatibility to the wind I would rip out the whole fromlist part of __import__() and let the bytecode worry about the fromlist, basically making the import opcode call importlib.import_module().
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